Pharmacologic manipulation of ob expression in a dietary model of obesity

The interplay between leptin, glucocorticoids, and GLP1 regulates food intake and feeding behaviour

Nutritional, endocrine, and neurological signals converge in multiple brain centres to control feeding behaviour and food intake as part of the allostatic regulation of energy balance. Among the several neuroendocrine systems involved, the leptin, glucocorticoid, and glucagon-like peptide 1 (GLP1) systems have been extensively researched. Leptin is at the top hierarchical level since its complete absence is sufficient to trigger severe hyperphagia. Glucocorticoids are key regulators of the energy balance adaptation to stress and their sustained excess leads to excessive adiposity and metabolic perturbations. GLP1 participates in metabolic adaptation to food intake, regulating insulin secretion and satiety by parallel central and peripheral signalling systems. Herein, we review the brain and peripheral targets of these three hormone systems that integrate to regulate food intake, feeding behaviour, and metabolic homeostasis. We examine the functional relationships between leptin, glucocorticoids, and GLP1 at the central and peripheral levels, including the cross-regulation of their circulating levels and their cooperative or antagonistic actions at different brain centres. The pathophysiological roles of these neuroendocrine systems in dysregulated intake are explored in the two extremes of body adiposity - obesity and lipodystrophy - and eating behaviour disorders.

Association of leptin mRNA expression with meat quality trait in Tianfu black rabbits

Leptin is a hormone synthesized and secreted primarily in adipocyte which can help to regulate energy balance. In this experiment, three tissue samples of Tianfu black rabbits at four growing periods were selected. The expression levels of leptin gene in different tissues were detected by real-time fluorescence quantitative PCR. The correlation analysis showed that the correlation coefficient between the expression levels of leptin gene in perirenal fat and intramuscular fat content in 84-day-old male rabbits was 0.73 (p < 0.05); the correlation coefficients between the expression levels of leptin gene in left biceps femoris and intramuscular fat and 24-hour pH in 84-day-old male rabbits were 0.95 (p < 0.01) and 0.85 (p < 0.05), respectively. Besides, the correlation coefficient between the expression levels of leptin gene in male left biceps femoris and cooked meat rate was 0.83 (p < 0.05). According to the analysis results, we inferred the expression levels of leptin gene in Tianfu black rabbits can influence meat quality and the meat quality of high expression levels of leptin gene in Tianfu black rabbits is better. These results revealed the leptin gene may be one of the important candidate genes for meat quality traits of molecular markers.

POMC neurons expressing leptin receptors coordinate metabolic responses to fasting via suppression of leptin levels

Leptin is critical for energy balance, glucose homeostasis, and for metabolic and neuroendocrine adaptations to starvation. A prevalent model predicts that leptin’s actions are mediated through pro-opiomelanocortin (POMC) neurons that express leptin receptors (LEPRs). However, previous studies have used prenatal genetic manipulations, which may be subject to developmental compensation. Here, we tested the direct contribution of POMC neurons expressing LEPRs in regulating energy balance, glucose homeostasis and leptin secretion during fasting using a spatiotemporally controlled Lepr expression mouse model. We report a dissociation between leptin’s effects on glucose homeostasis versus energy balance in POMC neurons. We show that these neurons are dispensable for regulating food intake, but are required for coordinating hepatic glucose production and for the fasting-induced fall in leptin levels, independent of changes in fat mass. We also identify a role for sympathetic nervous system regulation of the inhibitory adrenergic receptor (ADRA2A) in regulating leptin production. Collectively, our findings highlight a previously unrecognized role of POMC neurons in regulating leptin levels.

ApoA-IV: current and emerging roles in intestinal lipid metabolism, glucose homeostasis, and satiety

Apolipoprotein A-IV (apoA-IV) is secreted by the small intestine on chylomicrons into intestinal lymph in response to fat absorption. Many physiological functions have been ascribed to apoA-IV, including a role in chylomicron assembly and lipid metabolism, a mediator of reverse-cholesterol transport, an acute satiety factor, a regulator of gastric function, and, finally, a modulator of blood glucose homeostasis. The purpose of this review is to update our current view of intestinal apoA-IV synthesis and secretion and the physiological roles of apoA-IV in lipid metabolism and energy homeostasis, and to underscore the potential for intestinal apoA-IV to serve as a therapeutic target for the treatment of diabetes and obesity-related disease.

Regulatory effects of resveratrol on glucose metabolism and T-lymphocyte subsets in the development of high-fat diet-induced obesity in C57BL/6 mice

High-fat diet (HFD)-induced obesity is often associated with immune dysfunction. Resveratrol (trans-3,5,4'-trihydroxystilbene), which has well-founded immunity-related beneficial properties, was used to elucidate the regulatory effect on glucose metabolism and T-lymphocyte subsets in the development of HFD-induced obesity. Resveratrol, being associated with decreases of plasma leptin and plasma lipids and the release of oxidative stress, significantly decreased the body weight and fat masses in HF mice after 26 weeks of feeding. Furthermore, resveratrol decreased the fasting blood glucose and fasting plasma insulin and increased the CD3(+)CD4(+)/CD3(+)CD8(+) subsets percentages and the regulatory T cells (Tregs) production after 13 and 26 weeks of feeding. The results indicate that resveratrol, as an effective supplement for HFD, maintained glucose homeostasis by activating the PI3K and SIRT1 signaling pathways. Moreover, resveratrol activated the Nrf2 signaling pathway-mediated antioxidant enzyme expression to alleviate inflammation by protecting against oxidative damage and T-lymphocyte subset-related chronic inflammatory response in the development of HFD-induced obesity.

Apolipoprotein A-IV: a protein intimately involved in metabolism

The purpose of this review is to summarize our current understanding of the physiological roles of apoA-IV in metabolism, and to underscore the potential for apoA-IV to be a focus for new therapies aimed at the treatment of diabetes and obesity-related disorders. ApoA-IV is primarily synthesized by the small intestine, attached to chylomicrons by enterocytes, and secreted into intestinal lymph during fat absorption. In circulation, apoA-IV is associated with HDL and chylomicron remnants, but a large portion is lipoprotein free. Due to its anti-oxidative and anti-inflammatory properties, and because it can mediate reverse-cholesterol transport, proposed functions of circulating apoA-IV have been related to protection from cardiovascular disease. This review, however, focuses primarily on several properties of apoA-IV that impact other metabolic functions related to food intake, obesity, and diabetes. In addition to participating in triglyceride absorption, apoA-IV can act as an acute satiation factor through both peripheral and central routes of action. It also modulates glucose homeostasis through incretin-like effects on insulin secretion, and by moderating hepatic glucose production. While apoA-IV receptors remain to be conclusively identified, the latter modes of action suggest that this protein holds therapeutic promise for treating metabolic disease.

PACAP neurons in the hypothalamic ventromedial nucleus are targets of central leptin signaling

The adipose-derived hormone, leptin, was discovered over 10 years ago, but only now are we unmasking its downstream pathways which lead to reduced energy intake (feeding) and increased energy expenditure (thermogenesis). Recent transgenic models have challenged the long-standing supposition that the hypothalamic arcuate nucleus (Arc) is omnipotent in the central response to leptin, and research focus is beginning to shift to examine roles of extra-arcuate sites. Dhillon et al. (2006) demonstrated that targeted knock out of the signaling form of the leptin receptor (lepr-B) in steroidogenic factor 1 (SF-1) cells of the hypothalamic ventromedial nucleus (VMN) produces obesity of a similar magnitude to the pro-opiomelanocortin (POMC)-driven lepr-B deleted mouse, via a functionally distinct mechanism. These findings reveal that SF-1 cells of the VMN could be equally as important as POMC cells in mediating leptin's anti-obesity effects. However, the identification of molecular and cellular correlates of this relationship remains tantalizingly unknown. Here, we have shown that mRNA expression of the VMN-expressed neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) is regulated according to energy status and that it exerts catabolic effects when administered centrally to mice. Furthermore, we have shown that SF-1 and PACAP mRNAs are colocalized in the VMN, and that leptin signaling via lepr-B is required for normal PACAP expression in these cells. Finally, blocking endogenous central PACAP signaling with the antagonist PACAP(6-38) markedly attenuates leptin-induced hypophagia and hyperthermia in vivo. Thus, it appears that PACAP is an important mediator of central leptin effects on energy balance.

Differential activation of the sympathetic innervation of adipose tissues by melanocortin receptor stimulation

Melanocortins are implicated in the control of energy intake/expenditure. Centrally administered melanotan II (MTII), a synthetic melanocortin 3/4-receptor agonist, decreases adiposity beyond that accountable by food intake decreases. Melanocortin-4 receptor (MC4-R) mRNA is expressed on sympathetic nervous system (SNS) outflow neurons to white adipose tissue (WAT) in Siberian hamsters, suggesting a role in lipid mobilization. Therefore, we tested whether third ventricular injections of MTII increased sympathetic drive to WAT and interscapular brown adipose tissue (IBAT) using norepinephrine turnover (NETO) as a measure of sympathetic drive. We also tested for MTII-induced changes in lipolysis-related WAT gene expression (beta3-adrenoceptors, hormone sensitive lipase) and IBAT thermogenesis (beta3-adrenoceptor, uncoupling protein-1). Finally, we tested whether third ventricularly injected MTII, a highly selective MC4-R agonist (cyclo[beta-Ala-His-D-Phe-Arg-Trp-Glu]NH2) increased or agouti-related protein decreased IBAT temperature in hamsters implanted with sc IBAT temperature transponders. Centrally administered MTII provoked differential sympathetic drives to WAT and IBAT (increased inguinal WAT, dorsosubcutaneous WAT and IBAT NETO, but not epididymal WAT and retroperitoneal WAT NETO). MTII also increased circulating concentrations of the lipolytic products free fatty acids and glycerol but not plasma catecholamines, suggesting lipid mobilization via WAT SNS innervation and not via adrenal medullary catecholamines. WAT or IBAT gene expression was largely unaffected by acute MTII treatment, but IBAT temperature was increased by MTII and the MC4-R agonist and decreased by agouti-related protein. Collectively, this is the first demonstration of central melanocortin agonist stimulation of WAT lipolysis through the SNS and confirms melanocortin-induced changes in BAT thermogenesis.

Neuropeptidomics to study peptide processing in animal models of obesity

Neuropeptidomics is the analysis of the neuropeptides present in a tissue extract. Most neuropeptidomic studies use mass spectrometry to detect and identify the peptides, which provides information on the precise posttranslationally modified form of each peptide. Quantitative peptidomics uses isotopic labels to compare the levels of peptides in extracts from two different samples. This technique is ideal for examining neuropeptide levels in a variety of systems and is especially suited for studies of mice lacking peptide-processing enzymes. This review is focused on the neuropeptidomics technique and its application to the analysis of mice with a mutation that inactivates carboxypeptidase E, a critical enzyme in the biosynthesis of many neuroendocrine peptides. Mice without carboxypeptidase E activity are overweight, and a key question is the identification of the peptide or peptides responsible. The quantitative peptidomics approach has provided some insights toward the answer to this question.

Dietary n-3 fatty acids affect mRNA level of brown adipose tissue uncoupling protein 1, and white adipose tissue leptin and glucose transporter 4 in the rat

We examined the effect of dietary fats rich in n-3 polyunsaturated fatty acids (PUFA) on mRNA levels in white and brown adipose tissues in rats. Four groups of rats were fed on a low-fat diet (20 g safflower oil/kg) or a high-fat diet (200 g/kg) containing safflower oil, which is rich in n-6 PUFA (linoleic acid), or perilla (α-linolenic acid) or fish oil (eicosapentaenoic and docosahexaenoic acids), both of which are rich in n-3 PUFA, for 21 d. Energy intake was higher in rats fed on a high-safflower-oil diet than in those fed on low-fat or high-fish-oil diet, but no other significant differences were detected among the groups. Perirenal white adipose tissue weight was higher and epididymal white adipose tissue weight tended to be higher in rats fed on a high-safflower-oil diet than in those fed on a low-fat diet. However, high-fat diets rich in n-3 PUFA, compared to a low-fat diet, did not increase the white adipose tissue mass. High-fat diets relative to a low-fat diet increased brown adipose tissue uncoupling protein 1 mRNA level. The increases were greater with fats rich in n-3 PUFA than with n-6 PUFA. A high-safflower-oil diet, compared to a low-fat diet, doubled the leptin mRNA level in white adipose tissue. However, high-fat diets rich in n-3 PUFA failed to increase it. Compared to a low-fat diet, high-fat diets down-regulated the glucose transporter 4 mRNA level in white adipose tissue. However, the decreases were attenuated with high-fat diets rich in n-3 PUFA. It is suggested that the alterations in gene expression in adipose tissue contribute to the physiological activities of n-3 PUFA in preventing body fat accumulation and in regulating glucose metabolism in rats.

Dihydrotestosterone treatment results in obesity and altered lipid metabolism in orchidectomized mice

OBJECTIVE

To determine the role of androgen receptor (AR) activation for adipose tissue metabolism. Sex steroids are important regulators of adipose tissue metabolism in men. Androgens may regulate the adipose tissue metabolism in men either directly by stimulation of the AR or indirectly by aromatization of androgens into estrogens and, thereafter, by stimulation of the estrogen receptors. Previous studies have shown that estrogen receptor alpha stimulation results in reduced fat mass in men.

RESEARCH METHODS AND PROCEDURES

Orchidectomized mice were treated with the non-aromatizable androgen 5alpha-dihydrotestosterone (DHT), 17beta-estradiol, or vehicle. Vo(2), Vco(2), resting metabolic rate, locomotor activity, and food consumption were measured. Furthermore, changes in hepatic gene expression were analyzed.

RESULTS

DHT treatment resulted in obesity, associated with reduced energy expenditure and fat oxidation. In contrast, DHT did not affect food consumption or locomotor activity. Furthermore, DHT treatment resulted in increased high-density lipoprotein-cholesterol and triglyceride levels associated with markedly decreased 7alpha-hydroxylase gene expression, indicating decreased bile acid production.

DISCUSSION

We showed that AR activation results in obesity and altered lipid metabolism in orchidectomized mice. One may speculate that AR antagonists might be useful in the treatment of obesity in men.

Early dietary intervention: long-term effects on blood pressure, brain neuropeptide Y, and adiposity markers

Early life nutrition impacts on subsequent risk of obesity and hypertension. Several brain chemicals responsible for both feeding and cardiovascular regulation are altered in obesity. We examined effects of early postnatal overnutrition on blood pressure, brain neuropeptide Y (NPY), and adiposity markers. Rat pup litters were adjusted to either 3 or 12 male animals (overnutrition and control, respectively) on day 1 of life. After weaning, rats were given either a palatable high-fat diet or standard chow. Smaller litter pups were significantly heavier by 17 days of age. By 16 wk, the effect of litter size was masked by that of diet, postweaning. Small and normal litter animals fed a high-fat diet had similar increases in body weight, plasma insulin, leptin, and adiponectin concentrations, leptin mRNA, and fat masses relative to chow-fed animals. An increase in 11beta-hydroxysteroid dehydrogenase-1 mRNA in white adipose tissue, and a decrease in uncoupling protein-1 mRNA in brown adipose tissue in both small litter groups at 16 wk of age, may represent a programming effect of the altered litter size. NPY concentration in the paraventricular nucleus of the hypothalamus was reduced in high fat-fed groups. Blood pressure was significantly elevated at 13 wk in high-fat-fed animals. This study demonstrates that overnourishment during early postnatal development leads to profound changes in body weight at weaning, which tended to abate with maturation. Thus the effects of long-term dietary intervention postweaning can override those of litter size-induced obesity.

Apolipoprotein A-IV, food intake, and obesity

Apolipoprotein A-IV (apo A-IV) is secreted by the intestine associated with chylomicron. Intestinal apo A-IV synthesis is stimulated by fat absorption, which is probably mediated by chylomicron formation. The stimulation of apo A-IV synthesis in the jejunum and ileum is attenuated by intravenous leptin infusion. Intestinal apo A-IV synthesis is also stimulated by a factor from the ileum, probably peptide tyrosine-tyrosine (PYY), which has been demonstrated to affect satiety. Apo A-IV has been proposed to physiologically control food intake, a function not shared by apo A-I, and this inhibitory effect is centrally mediated. Recently, apo A-IV was demonstrated in the hypothalamus. The hypothalamic apo A-IV level was reduced by food deprivation and restored by lipid feeding. Intracerebroventricular administration of apo A-IV antiserum increased feeding and decreased the hypothalamic apo A-IV mRNA level, implying that feeding is normally limited by endogenous apo A-IV. Central administration of neuropeptide Y (NPY) significantly increased hypothalamic apo A-IV mRNA levels in a dose-dependent manner. The stimulation of intestinal synthesis and secretion of apo A-IV by lipid absorption are rapid; thus, apo A-IV is capable of short-term regulation of food intake. Evidence also suggests apo A-IV's involvement in the long-term regulation of food intake and body weight. Chronic ingestion of high fat blunts the hypothalamic apo A-IV response to lipid feeding and may therefore explain why chronic intake of high fat predisposes animals and humans to obesity.

Ingested fat and satiety

Apolipoprotein A-IV (apo A-IV) is secreted by the intestine associated with chylomicron. Intestinal apo A-IV synthesis is stimulated by fat absorption, probably mediated by chylomicron formation. The stimulation of apo A-IV synthesis in the jejunum and ileum is attenuated by intravenous leptin infusion. Intestinal apo A-IV synthesis is also stimulated by a factor from the ileum, probably peptide tyrosine-tyrosine (PYY), which has been demonstrated to affect satiety. Apo A-IV has been proposed to physiologically control food intake, and this inhibitory effect is centrally mediated. Recently, apo A-IV was demonstrated in the hypothalamus. The hypothalamic apo A-IV level was reduced by food deprivation and restored by lipid feeding. Intracerebroventricular administration of apo A-IV antiserum stimulated feeding and decreased the hypothalamic apo A-IV mRNA level, implying that feeding is normally limited by endogenous apo A-IV. Central administration of neuropeptide Y (NPY) significantly increased hypothalamic apo A-IV mRNA levels in a dose-dependent manner. The stimulation of intestinal synthesis and secretion of apo A-IV by lipid absorption are rapid; thus, apo A-IV is capable of short-term regulation of food intake. Evidence also suggests apo A-IV's involvement in long-term regulation of food intake and bodyweight. The chronic ingestion of high fat blunts the intestinal apo A-IV response to lipid feeding and may therefore explain why chronic intake of high fat predisposes animals and humans to obesity.

Genetic vulnerability to diet-induced obesity in the C57BL/6J mouse: physiological and molecular characteristics

The development of the metabolic syndrome in an increasing percentage of the populations of Western societies, particularly in the United States, requires valid models for establishing basic biochemical changes and performing preclinical studies on potential drug targets. The C57BL/6J mouse has become an important model for understanding the interplay between genetic background and environmental challenges such as high-fat/high-calorie diets that predispose to the development of the metabolic syndrome. This review highlights metabolic and signal transduction features that are altered during the course of disease progression, many of which mirror the human situation.

Leptin and insulin modulate nutrient partitioning and weight loss in ob/ob mice through regulation of long-chain fatty acid uptake by adipocytes

Leptin treatment of ob/ob mice leads to weight loss appreciably greater than that in pair-fed mice. To test whether this "extra" weight loss is mediated by leptin-induced alterations in nutrient partitioning, the effects in ob/ob mice of subcutaneous leptin infusion (500 ng/h for <or=21 d) on adipocyte fatty acid uptake and transporter gene expression were examined. Mice were initially hyperinsulinemic (5.25 +/- 1.57 nmol/L). Plasma insulin decreased by 55 +/- 10% within 8 h of leptin infusion, declining progressively to normal by d 14. The V(max) for saturable adipocyte fatty acid uptake fell from 31.1 +/- 5.6 to 25.2 +/- 4.0 pmol/(s. 50000 cells) (P < 0.05) by 24 h, and to a normal rate (8.0 +/- 0.8 pmol/(s. 50000 cells) by d 21 (P > 0.5 vs. normal C57BL/6J controls). Adipocyte mRNA levels for plasma membrane fatty acid binding protein and fatty acid translocase, putative fatty acid transporters that are up-regulated three- to fourfold in adipocytes from ob/ob mice, had also normalized by d 21. The initial changes in V(max) preceded decreases in food intake and body weight by at least 24 h. In pair-fed mice, insulin levels, V(max) and body weight all declined more slowly than in leptin-treated mice, and all remained significantly elevated compared with normal values at d 21. The data suggest that insulin up-regulates and leptin down-regulates adipocyte fatty acid uptake, leading to alterations in fatty acid partitioning that affect adiposity.

Adrenalectomy stimulates hypothalamic proopiomelanocortin expression but does not correct diet-induced obesity

BACKGROUND

Elevated glucocorticoid production and reduced hypothalamic POMC mRNA can cause obese phenotypes. Conversely, adrenalectomy can reverse obese phenotypes caused by the absence of leptin, a model in which glucocorticoid production is elevated. Adrenalectomy also increases hypothalamic POMC mRNA in leptin-deficient mice. However most forms of human obesity do not appear to entail elevated plasma glucocorticoids. It is therefore not clear if reducing glucocorticoid production would be useful to treat these forms of obesity. We hypothesized that adrenalectomy would increase hypothalamic POMC mRNA and reverse obese phenotypes in obesity due to a high-fat diet as it does in obesity due to leptin deficiency.

RESULTS

Retired breeder male mice were placed on a high-fat diet or a low-fat diet for two weeks, then adrenalectomized or sham-adrenalectomized. The high-fat diet increased body weight, adiposity, and plasma leptin, led to impaired glucose tolerance, and slightly stimulated hypothalamic proopiomelanocortin (POMC) expression. Adrenalectomy of mice on the high-fat diet significantly reduced plasma corticosterone and strikingly increased both pituitary and hypothalamic POMC mRNA, but failed to reduce body weight, adiposity or leptin, although slight improvements in glucose tolerance and metabolic rate were observed.

CONCLUSION

These data suggest that neither reduction of plasma glucocorticoid levels nor elevation of hypothalamic POMC expression is effective to significantly reverse diet-induced obesity.

Peripheral injection of risperidone, an atypical antipsychotic, alters the bodyweight gain of rats

1. Risperidone is an atypical antipsychotic drug that possesses 5-hydroxytryptamine 5-HT2 receptor antagonism combined with milder dopamine D2 receptor antagonism. 2. Excessive bodyweight gain is one of the side-effects of antipsychotics. Risperidone treatment causes a greater increase in the body mass of patients than treatment with conventional antipsychotics, such as haloperidol. Therefore, the present study was undertaken in order to address the aetiology of the risperidone-induced bodyweight change in rats by examining the expression of leptin, an appetite-regulating hormone produced in white adipose tissue (WAT), and uncoupling protein (UCP)-1, a substance promoting energy expenditure in the brown adipose tissues (BAT). 3. Eight-week-old male rats were injected subcutaneously with risperidone (0.005, 0.05 or 0.5 mg/kg) twice daily for 21 days. Both bodyweight and food intake were monitored daily. On day 21, rats were decapitated and their serum leptin and prolactin concentrations were measured. Expression levels of leptin, Ucp1 and beta3-adrenoceptor (beta3-AR) genes in WAT and BAT were quantified using real-time polymerase chain reaction amplification. 4. Injection of 0.005 mg/kg risperidone into rats increased food intake and the rate of bodyweight gain, as well as the augmentation of leptin gene expression in WAT. Injection of 0.05 mg/kg risperidone increased food intake and leptin gene expression in WAT, but the rate of bodyweight gain was not affected. Injection of 0.5 mg/kg risperidone caused a reduction in bodyweight gain, as well as enhanced Ucp1 gene expression in BAT and serum prolactin concentrations. The serum leptin concentration and beta3-AR gene expression in WAT and BAT were not affected by injection of 0.5 mg/kg risperidone. 5. Although the changes in food intake observed in risperidone-injected rats were rationalized neither by serum leptin nor prolactin concentrations, the reduction in the rate of bodyweight gain following injection of 0.5 mg/kg can be explained, in part, by increased energy expenditure, as revealed by the remarkable increase in the UCP-1 mRNA expression level in BAT. The role of leptin in risperidone-induced alterations in bodyweight gain remain to be clarified.

Intracellular fatty acid downregulates ob gene expression in 3T3-L1 adipocytes

The effect of intracellular free fatty acid (FFA) accumulation on ob gene expression in adipocytes was examined. In fully differentiated 3T3-L1 adipocytes, triacsin C, a specific acyl CoA synthetase inhibitor with a K(i) of 8.97 microM, inhibited ob gene expression by 20% at 5 x 10(-5)M. At this concentration, triacsin C induced accumulation of intracellular FFA. Treatment with both chylomicron and triacsin C reduced ob gene expression more than treatment with triacsin C alone. Treatment with 2-bromopalmitate, a poorly metabolizable palmitate analog, reduced ob gene expression by 50% at 10(-4)M, but palmitate at the same concentration had no effect. This is the first demonstration that the ob gene is downregulated by intracellular FFA accumulation, thereby raising the possibility that ob product is regulated in response to lipolysis.

Deconstructing and reconstructing obesity-induced diabetes (diabesity) in mice

Obesity-driven type 2 diabetes (diabesity) involves complex genetic and environmental interactions to trigger disease. Here, we combine variable numbers of known quantitative trait loci (QTL) for obesity and diabetes contributed by New Zealand Obese (NZO/HlLt) and Nonobese Nondiabetic (NON/Lt) strains in the form of 10 interval-directed recombinant congenic strains (RCS), with NON/Lt as the background strain, to dissect the genetic interactions involved. All 10 RCS gain significantly more weight than the NON parental strain, but none are as obese as the parental, diabetes-prone NZO. Diabetes development in these RCS at F12 ranges between 0 and 100%, depending on genetic constitution. RCS-2, -1, and -10 represent a step-wise increase in numbers of specific diabetogenic QTL, resulting in a step-wise increase in diabetes incidence. RCS-10 recreates the 100% incidence seen in (NZOxNON)F1 males, but with less weight gain. Similarly, RCS-6, -7, -8, and -9 represent diabetes-prone strains with different combinations of diabetogenic QTL. RCS-3, -4, and -5 represent obese strains that do not transit to diabetes. Because these obesity and diabetes syndromes reflect different collections of QTL, rather than null mutations in the leptin or leptin receptor genes, they are extremely relevant as models for the polygenic obesity/diabesity syndromes in humans.

Aromatase-deficient (ArKO) mice accumulate excess adipose tissue

Aromatase is the enzyme which catalyses the conversion of C19 steroids into C18 estrogens. We have generated a mouse model wherein the Cyp19 gene, which encodes aromatase, has been disrupted, and hence, the aromatase knockout (ArKO) mouse cannot synthesise endogenous estrogens. We examined the consequences of estrogen deficiency on accumulation of adipose depots in male and female ArKO mice, observing that these animals progressively accrue significantly more intra-abdominal adipose tissue than their wildtype (WT) litter mates, reflected in increased adipocyte volume and number. This increased adiposity was not due to hyperphagia or reduced resting energy expenditure, but was associated with reduced spontaneous physical activity levels, reduced glucose oxidation, and a decrease in lean body mass. Elevated circulating levels of leptin and cholesterol were present in 1-year-old ArKO mice compared to WT controls, as were elevated insulin levels, although blood glucose was unchanged. Associated with these changes, the livers of ArKO animals were characterised by a striking accumulation of lipid droplets. Our findings demonstrate an important role for estrogen in the maintenance of lipid homeostasis in both males and females.

Adrenoceptors, uncoupling proteins, and energy expenditure

Interest in the biology of adipose tissue has undergone a revival in recent years with the discovery of a host of genes that contribute to the regulation of satiety and metabolic rate. The catecholamines have long been known to be key modulators of adipose tissue lipolysis and the hydrolysis of triglyceride energy stores. However, more recent efforts to understand the role of individual adrenergic receptor subtypes expressed in adipocytes and their signal transduction pathways have revealed a complexity not previously appreciated. Combined with this interest in the modulation of adipocyte metabolism is a renewed focus upon brown adipose tissue and the mechanisms of whole body thermogenesis in general. The discovery of novel homologs of the brown fat uncoupling protein (UCP) such as UCP2 and UCP3 has provoked intensive study of these mitochondrial proteins and the role that they play in fuel metabolism. The story of the novel UCPs has proven to be intriguing and still incompletely understood. Here, we review the status of adipose tissue from inert storage depot to endocrine organ, interesting signal transduction pathways triggered by beta-adrenergic receptors in adipocytes, the potential of these receptors for discriminating and coordinated metabolic regulation, and current views on the role of UCP2 and UCP3 based on physiological studies and gene knockout models.

Effect of leptin on intestinal apolipoprotein AIV in response to lipid feeding

We determined apolipoprotein AIV (apo AIV) content in intestinal epithelial cells using immunohistochemistry when leptin was administered intravenously. Most of the apo AIV immunoreactivity in the untreated intestine was located in the villous cells as opposed to the crypt cells. Regional distribution of apo AIV immunostaining revealed low apo AIV content in the duodenum and high content in the jejunum that gradually decreases caudally toward the ileum. Intraduodenal infusion of lipid (4 h) significantly increased apo AIV immunoreactivity in the jejunum and ileum. Simultaneous intravenous leptin infusion plus duodenal lipid infusion markedly suppressed apo AIV immunoreactivity. Duodenal lipid infusion increased plasma apo AIV significantly (measured by ELISA), whereas simultaneous leptin infusion attenuated the increase. These findings suggest that leptin may regulate circulating apo AIV by suppressing apo AIV synthesis in the small intestine.

Up-regulation of a thermogenesis-related gene (UCP1) and down-regulation of PPARgamma and aP2 genes in adipose tissue: possible features of the antiobesity effects of a beta3-adrenergic agonist

A number of experiments have demonstrated the antiobesity effects of beta(3)-adrenergic receptor stimulation by promoting thermogenesis and/or lipolysis. While many studies have been performed in order to develop beta(3)-adrenergic agonists as a novel strategy in the management of obesity, more information is needed about the mechanisms involved in thermogenesis and the actions of these drugs on adipocyte differentiation. To address this, the possible thermogenic and antiadipogenic properties of Tertatolol, a beta(3)-adrenergic agonist, in a diet-induced obesity model has been tested. Animals fed on a high-fat diet gained more weight and fat mass as compared with control and high-fat fed animals treated with Tertatolol. A RT-PCR was carried out in white adipose tissue specific genes involved in thermogenesis such as uncoupling proteins (UCPs) and adipogenesis such as peroxisome proliferator-activated receptor (PPARgamma2), retinoid receptors (RXRalpha/RARalpha), and fatty acid binding protein (aP2). Levels of UCP1 mRNA were augmented in the Tertatolol-treated group as compared to non-treated high-fat fed animals, while the beta(3)-adrenergic agonist treatment significantly decreased the expression levels of aP2 and transcription factors such as PPARgamma2 and the ratio RXRalpha/RARalpha as compared to obese rats. Altogether these data suggest that the antiobesity effects of beta(3)-adrenergic agonists are not limited to the promotion of thermogenesis and/or lipolysis and support the implication that these beta(3)-adrenergic agonists also affect fat deposition by impairing adipogenesis in white adipose tissue (WAT).

Effect of the beta(3)-adrenergic agonist Cl316,243 on functional differentiation of white and brown adipocytes in primary cell culture

We investigated the effect of the specific beta(3)-adrenergic receptor agonist CL 316,243 (CL) on proliferation and functional differentiation of the Siberian hamster (Phodopus sungorus) white and brown preadipocytes in primary cell culture. Proliferation of both white and brown preadipocytes was stimulated by a general beta-adrenergic agonist (isoproterenol) but not by CL. Lipolysis of differentiated white and brown adipocytes was stimulated similarly by CL with maximum effect at 10 nM. Thermogenic properties of cells were assessed by immunodetection of UCP-1, the brown adipocyte specific uncoupling protein, and measurement of cytochrome c oxidase (COx) activity as an index of mitochondrial capacity. UCP-1 content was largely increased by CL in BAT but not in WAT cultures. Basal UCP-2 mRNA levels were similar in WAT and BAT cultures and increased by both CL and isoproterenol. COx activity of BAT cultures was twice as high as that of WAT cultures but in neither cell culture system could it be increased by beta-adrenergic stimulation. We suggest (i) that white and brown preadipocyte proliferation is increased in vitro via beta1 or beta(2), but not beta(3)-adrenergic pathways, (ii) that white and brown preadipocytes represent different cell types, and (iii) that in vitro beta-adrenergic stimulation it is not sufficient to induce complete thermogenic adaptation of brown adipocytes.

CCAAT/enhancer-binding protein alpha is required for transcription of the beta 3-adrenergic receptor gene during adipogenesis

The beta(3)-adrenergic receptor (beta(3)AR) is expressed predominantly in adipocytes, and it plays a major role in regulating lipolysis and adaptive thermogenesis. Its expression in a variety of adipocyte cell models is preceded by the appearance of CCAAT/enhancer-binding protein alpha (C/EBP alpha), which has been shown to regulate a number of other adipocyte-specific genes. Importantly, it has been demonstrated that several adipocyte cell lines that fail to express C/EBP alpha exhibit reduced insulin sensitivity, despite an apparent adipogenic phenotype. Here we show that transcription and function of the beta(3)AR correlates with C/EBP alpha expression in these adipocyte models. A 5.13-kilobase pair fragment of the mouse beta(3)AR promoter was isolated and sequenced. This fragment conferred a 50-fold increase in luciferase reporter gene expression in adipocytes. Two putative C/EBP binding sites exist at -3306 to -3298 and at -1462 to -1454, but only the more distal site is functional. Oligonucleotides corresponding to both the wild-type and mutated -3306 element were inserted upstream of a thymidine kinase luciferase construct. When cotransfected in fibroblasts with a C/EBP alpha expression vector, reporter gene expression increased 3-fold only in the wild-type constructs. The same mutation, when placed into the intact 5.13-kilobase pair promoter, reduced promoter activity in adipocytes from 50-fold to <10-fold. Electrophoretic mobility shift analysis demonstrated that the site at -3306 generated a specific protein-oligonucleotide complex that was supershifted by C/EBP alpha antibody, while a probe corresponding to a putative site at -1462 did not. These results define C/EBP alpha as a key transcriptional regulator of the mouse beta(3)AR gene during adipogenesis.

Direct binding of activated c-Src to the beta 3-adrenergic receptor is required for MAP kinase activation

Both beta(2)- and beta(3)-adrenergic receptors (ARs) are able to activate the extracellular signal-regulated kinase (ERK) pathway. We previously showed that c-Src is required for ERK activation by beta(2)AR and that it is recruited to activated beta(2)AR through binding of the Src homology 3 (SH3) domain to proline-rich regions of the adapter protein beta-arrestin1. Despite the absence of sites for phosphorylation and beta-arrestin binding, ERK activation by beta(3)AR still requires c-Src. Agonist activation of beta(2)AR, but not beta(3)AR, led to redistribution of green fluorescent protein-tagged beta-arrestin to the plasma membrane. In beta-arrestin-deficient COS-7 cells, beta-agonist-dependent co-precipitation of c-Src with the beta(2)AR required exogenous beta-arrestin, but activated beta(3)AR co-precipitated c-Src in the absence or presence of beta-arrestin. ERK activation and Src co-precipitation with beta(3)AR also occurred in adipocytes in an agonist-dependent and pertussis toxin-sensitive manner. Protein interaction studies show that the beta(3)AR interacts directly with the SH3 domain of Src through proline-rich motifs (PXXP) in the third intracellular loop and the carboxyl terminus. ERK activation and Src co-precipitation were abolished in cells expressing point mutations in these PXXP motifs. Together, these data describe a novel mechanism of ERK activation by a G protein-coupled receptor in which the intracellular domains directly recruit c-Src.

Aromatase-deficient (ArKO) mice have a phenotype of increased adiposity

The aromatase-knockout (ArKO) mouse provides a useful model to examine the role that estrogens play in development and homeostasis in mammals. Lacking a functional Cyp19 gene, which encodes aromatase, the ArKO mouse cannot synthesize endogenous estrogens. We examined the adipose depots of male and female ArKO mice, observing that these animals progressively accumulate significantly more intraabdominal adipose tissue than their wild-type (WT) littermates, reflected in increased adipocyte volume at gonadal and infrarenal sites. This increased adiposity was not due to hyperphagia or reduced resting energy expenditure, but was associated with reduced spontaneous physical activity levels, reduced glucose oxidation, and a decrease in lean body mass. Elevated circulating levels of leptin and cholesterol were present in 1-year-old ArKO mice compared with WT controls, as were elevated insulin levels, although blood glucose levels were unchanged. Associated with these changes, a striking accumulation of lipid droplets was observed in the livers of ArKO animals. Our findings demonstrate an important role for estrogen in the maintenance of lipid homeostasis in both males and females.

Differential regulation of leptin expression and function in A/J vs. C57BL/6J mice during diet-induced obesity

Obesity-resistant (A/J) and obesity-prone (C57BL/6J) mice were weaned onto low-fat (LF) or high-fat (HF) diets and studied after 2, 10, and 16 wk. Despite consuming the same amount of food, A/J mice on the HF diet deposited less carcass lipid and gained less weight than C57BL/6J mice over the course of the study. Leptin mRNA was increased in white adipose tissue (WAT) in both strains on the HF diet but to significantly higher levels in A/J compared with C57BL/6J mice. Uncoupling protein 1 (UCP1) and UCP2 mRNA were induced by the HF diet in brown adipose tissue (BAT) and WAT of A/J mice, respectively, but not in C57BL/6J mice. UCP1 mRNA was also significantly higher in retroperitoneal WAT of A/J compared with C57BL/6J mice. The ability of A/J mice to resist diet-induced obesity is associated with a strain-specific increase in leptin, UCP1, and UCP2 expression in adipose tissue. The findings indicate that the HF diet does not compromise leptin-dependent regulation of adipocyte gene expression in A/J mice and suggest that maintenance of leptin responsiveness confers resistance to diet-induced obesity.

Hypothalamic c-fos-like immunoreactivity in high-fat diet-induced obese and resistant mice

Some C57Bl/6 mice become obese, whereas others remain lean when raised on a high-fat diet. The mechanisms underlying this interindividual susceptibility to diet-induced obesity remain unknown. Because hypothalamus plays a major role in the regulation of body weight, this study was conducted to identify the differences of hypothalamic neuronal activity between diet-induced obese and diet-resistant mice. Using c-fos as a marker, this study showed that diet-induced obese mice significantly increased c-fos-like immunoreactive neurons in the dorsal part of lateral hypothalamus (+183%) and dorsomedial hypothalamic nucleus (+87.5%) compared with diet-resistant mice. Furthermore, switching from high fat to low fat, or high n-3 polyunsaturated fatty acid diet, significantly decreased body weight gain (-35.7% and -31.0%), overall fat storage (-63.4% and -59.6%), and c-fos-like immunoreactive neurons in the dorsal part of lateral hypothalamus (-76.5% and -64.7%) and dorsomedial hypothalamic nucleus (-73.3% and -56.7%) in diet-induced obese mice, respectively. The present study also showed that the ratio of serum leptin/fat mass was threefold higher in the diet-resistant mice than in the diet-induced obese mice, which may be responsible for the less fat storage in the diet-resistant mice. The current data further confirm that the increased neuronal activity in the key autonomic regulatory centers may contribute to the excessive fat storage in diet-induced obese mice. Moreover, both high-fat diet-induced excessive fat storage and the altered hypothalamic neuronal activity may be largely corrected by reducing dietary fat content or replacing it with non-obesogenic fat.

Regulation of PPARgamma but not obese gene expression by dietary fat supplementation

Leptin, the product of the obese gene, and peroxisome proliferator activated receptor gamma (PPARgamma) are important regulators of energy metabolism, adipogenesis, and immune function. In rodent models, both genes seem to respond at the mRNA and/or protein levels to dietary fat consumption. To determine the effect(s) of dietary saturated and polyunsaturated fatty acids on the expression (mRNA abundance) of these genes, adipose tissue was obtained from pigs fed three different dietary fat sources. Corn-soybean meal diets containing no added fat (NO, control) or 10% beef tallow (BT), safflower oil (SO), or fish oil (FO) were fed ad libitum (n = 12) for 12 weeks. The abundance of obese, PPARgamma1, and PPARgamma2 mRNA was quantified relative to 18S rRNA using ribonuclease protection assays. The gain:feed ratio was improved (P < 0.05) 21% by all fats with a corresponding reduction (P < 0.05) in feed intake. Relative to pigs fed NO, serum total cholesterol was increased (P < 0.01) in pigs fed BT and triglyceride and nonesterified fatty acid concentrations were increased (P < 0.01) by all supplemental fats. Serum insulin was increased (P < 0.10) only by SO. Neither obese nor PPARgamma1 mRNA abundance were responsive to added fat (P > 0.15). However, the abundance of PPARgamma2 mRNA was increased fourfold by SO compared with the NO diet. These data indicate that the abundance of obese mRNA is independent of dietary fat consumption per se, whether saturated or unsaturated, when feed consumption is reduced due to greater dietary caloric density. Furthermore, we provide evidence that expression of the PPARgamma2 gene in porcine adipose tissue is selectively responsive to SO (presumably linoleic acid, 18:2n-6).

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

OBJECTIVE

The anti-obesity properties of a new beta3-adrenergic agonist (Trecadrine) were examined in a diet-induced obesity model, including the effects on OB and uncoupling protein (UCP-1 and -2) gene expression.

MEASUREMENTS

Control rats and cafeteria-fed rats were treated with placebo or Trecadrine for 35 days. Leptin and UCP (1 and 2) mRNA levels were determined by reverse transcription-polymerase chain reaction (RT-PCR) methodology in adipose tissue and gastrocnemius muscle.

RESULTS

Animals fed a cafeteria diet increased body weight, fat content, white adipose tissue (WAT), brown adipose tissue (BAT) weights and oxygen consumption in relation to lean controls. A rise in plasma leptin, WAT OB gene expression as well as circulating free fatty acids levels was found in obese rats as compared with lean controls. Trecadrine administration to cafeteria-fed animals decreased fat content, WAT weight, circulating leptin and fatty acids concentrations, and WAT OB gene expression, reaching comparable values to lean controls, while WAT O2 consumption was increased in these animals. Also, an increase in BAT UCP1 mRNA levels was found through a two-way analysis of variance in control and obese animals after Trecadrine administration. Gastrocnemius muscle UCP2 gene expression was reduced in lean Trecadrine-treated and diet-induced obese animals as compared to controls, while an increase was found in cafeteria-fed animals after Trecadrine administration. A negative correlation between WAT O2 consumption and UCP2 expression was found in control animals, but not in the cafeteria-fed groups, suggesting a differential response to the beta3-adrenergic compound in lean and obese animals, which is in agreement with the reported statistical interactions between obesity and Trecadrine administration found for WAT O2 consumption and muscle UCP2 expression, as well as for plasma leptin and WAT leptin expression.

CONCLUSION

The new beta3-adrenergic agonist, Trecadrine, decreases fat content and increases gastrocnemius muscle UCP2 gene expression in a diet-induced obesity model. This sheds additional light on the action mechanism of compounds with affinity for beta3-adrenoceptors and other potential anti-obesity agents.

Mutation of the RIIbeta subunit of protein kinase A differentially affects lipolysis but not gene induction in white adipose tissue

Targeted disruption of the RIIbeta subunit of protein kinase A (PKA) produces lean mice that resist diet-induced obesity. In this report we examine the effects of the RIIbeta knockout on white adipose tissue physiology. Loss of RIIbeta is compensated by an increase in the RIalpha isoform, generating an isoform switch from a type II to a type I PKA. Type I holoenzyme binds cAMP more avidly and is more easily activated than the type II enzyme. These alterations are associated with increases in both basal kinase activity and the basal rate of lipolysis, possibly contributing to the lean phenotype. However, the ability of both beta(3)-selective and nonspecific beta-adrenergic agonists to stimulate lipolysis is markedly compromised in mutant white adipose tissue. This defect was found in vitro and in vivo and does not result from reduced expression of beta-adrenergic receptor or hormone-sensitive lipase genes. In contrast, beta-adrenergic stimulated gene transcription remains intact, and the expression of key genes involved in lipid metabolism is normal under both fasted and fed conditions. We suggest that the R subunit isoform switch disrupts the subcellular localization of PKA that is required for efficient transduction of signals that modulate lipolysis but not for those that mediate gene expression.

Does beta(3)-adrenoreceptor blockade attenuate acute exercise-induced reductions in leptin mRNA?

We investigated the effect of a single bout of exercise on leptin mRNA levels in rat white adipose tissue. Male Sprague-Dawley rats were randomly assigned to an exercise or control group. Acute exercise was performed on a rodent treadmill and was carried out to exhaustion, lasting an average of 85.5 +/- 1.5 min. At the end of exercise, soleus muscle and liver glycogen were reduced by 88% (P < 0.001). Acutely exercised animals had lower (P < 0.05) leptin mRNA levels in retroperitoneal but not epididymal fat, and this was independent of fat pad weight. To test the hypothesis that beta(3)-adrenergic-receptor stimulation was involved in the downregulation of leptin mRNA in retroperitoneal fat, a second experiment was performed in which rats were randomized into one of four groups: control, control + beta(3)-antagonist, exercise, and exercise + beta(3)-antagonist. A highly selective beta(3)-antagonist (SR-59230A) or vehicle was given by gavage 30 min before exercise or control experiment. Exercise consisted of 55 min of treadmill running, sufficient to reduce liver and muscle glycogen by 70 and 80%, respectively (both P < 0.0001). Again, acute exercise reduced leptin mRNA in retroperitoneal fat (exercise vs. control; P < 0.05), but beta(3)-antagonism blocked this effect (exercise + beta(3)-antagonist vs. control + beta(3)-antagonist; P = 0.42). Unexpectedly, exercise increased serum leptin. This would be consistent with the idea that there are releasable, preformed pools of leptin within adipocytes. We conclude that beta(3)-receptor stimulation is a mechanism by which acute exercise downregulates retroperitoneal adipose tissue leptin mRNA in vivo.

Sympathetic inhibition, leptin, and uncoupling protein subtype expression in normal fasting rats

To further investigate neural effects on leptin and uncoupling proteins (UCPs), we studied in vivo perturbations intended to block adrenergic input to peripheral tissues. We examined plasma leptin, leptin mRNA, and adipose and muscle UCP subtype mRNA in rats treated with alpha-methyl-p-tyrosine methyl ester (AMPT-ME), which inhibits catecholamine synthesis and 6-hydroxydopamine (6HDA), which is toxic to catecholinergic nerve terminals but, unlike AMPT-ME, does not enter the central nervous system. Intraperitoneal AMPT-ME, 250 mg/kg, was administered at 1800 and 0700 the following day, and rats were killed at 1200-1400. All rats were fasted with free access to water during this time. Intraperitoneal AMPT-ME increased plasma leptin by 15-fold, increased interscapular brown adipose tissue (IBAT) and epididymal fat leptin mRNA by 2- to 2.5-fold, and also increased plasma insulin and glucose concentrations. Intraperitoneal AMPT-ME decreased IBAT UCP-3 mRNA to 40% of control, while it increased epididymal adipose UCP-3 mRNA approximately twofold. Intravenous AMPT-ME, 250 mg/kg, administered to conscious rats for 5 h decreased lumbar sympathetic nerve activity, increased plasma leptin (5.89 +/- 1.43 compared with 2.75 +/- 0.31 ng/ml in vehicle-treated rats, n = 7, P < 0.05), and decreased cardiac rate with no sustained change in blood pressure. Intraperitoneal 6HDA, 100 mg/kg, as a single dose at 1800, increased plasma leptin approximately twofold after 18-20 h, increased IBAT (but not epididymal fat) leptin mRNA by two- to threefold, and decreased IBAT UCP-3 mRNA to 30-40% of control. Neither AMPT-ME nor 6HDA significantly altered mRNA encoding gastrocnemius muscle UCP-3, IBAT UCP-1, or IBAT and epididymal UCP-2. In summary, AMPT-ME and 6HDA increased plasma leptin and upregulated leptin mRNA expression. AMPT-ME also resulted in complex tissue and subtype-specific modulation of adipose UCP mRNA. These data are consistent with interaction between leptin and sympathetic nerve activity (SNA) in regulation of fat cell energy utilization. However, the in vivo modulation of leptin and UCPs appears complex and, beyond a causal effect of SNA per se, may depend on concurrent changes in plasma insulin, glucose, and circulatory hemodynamics.

Effect of dietary fats differing in degree of unsaturation on gene expression in rat adipose tissue

To test the possibility that the type of dietary fat affects the expression of proteins involved in adipose tissue metabolism, levels of mRNA for lipoprotein lipase, leptin, glucose transporter 4, and uncoupling protein in adipose tissues were compared among rats fed a low-fat diet (2% safflower oil), and high-fat diets containing 20% saturated fat (palm oil) or unsaturated fat rich in linoleic acid (safflower oil) for 3 weeks. High-fat diets decreased the lipoprotein lipase mRNA level in epididymal but not in perirenal white adipose tissue, but increased it in brown adipose tissue. Leptin gene expression in perirenal white adipose tissue was significantly higher in rats fed high-fat diets than in those fed a low-fat diet. High-fat diets failed, however, to alter this parameter in epididymal white adipose tissue and interscapular brown adipose tissue. mRNA levels of glucose transporter 4, both in epididymal and perirenal white adipose tissues, were lower in rats fed high-fat diets than in those fed a low-fat diet. Uncoupling protein gene expression in interscapular brown adipose tissue was 2-3 times higher in rats fed high-fat diets than in those fed a low-fat diet. The abundance of mRNAs for lipoprotein lipase, leptin, glucose transporter 4 and uncoupling protein was, however, comparable between rats fed diets high in safflower and palm oil. We concluded that the high-fat diet influences gene expression of adipose tissue in a site-specific manner. The difference in the degree of unsaturation of dietary fats is rather irrelevant in modifying the level of mRNAs for proteins related to energy metabolism and expenditure in adipose tissue.

Leptin treatment increases suppressors of cytokine signaling in central and peripheral tissues

Leptin concentrations are elevated in the majority of obese individuals raising the possibility that leptin resistance contributes to their obesity. Peripheral leptin administration for 48 h caused a several-fold increase in mRNA encoding the suppressors of cytokine signaling SOCS-3 and CIS in hypothalamus and peripheral tissues. Paradoxically, CIS and SOCS-3 mRNAs are also elevated in the leptin-deficient ob/ob mouse. Forced expression of CIS in insulinoma cells prevented transactivation mediated by leptin. Thus tissues continuously exposed to leptin and/or other factors associated with obesity accumulate excessive amounts of SOCS-3 and CIS which could provide a potential mechanism for leptin resistance.

Leptin and its potential role in human obesity

Genetic studies in inbred obese mice have revealed the ob gene, its product leptin and the leptin receptor as important factors in the regulation of both appetite and energy expenditure. Treatment with recombinant leptin has resulted in a marked weight reduction in obese animals with ob gene mutations as well as in normal mice. Also mutations in the Ob receptor gene result in marked obesity in rodents. These data have given hope of new treatment options in obesity. Further support of leptin being involved in regulation of obesity in man comes from the observation that inactivating mutations in the human ob gene lead to profound early onset obesity. However, the role of leptin and its feedback system in man is still only partly revealed. This review focuses on our present knowledge and hypotheses about the leptin pathway in humans and its potential importance in the clinic of obesity.

Isoproterenol decreases leptin expression in adipose tissue of obese humans

OBJECTIVE

We investigated the effects of the non-selective beta-adrenergic agonist, isoproterenol (Iso), on leptin expression in human adipose tissue.

RESEARCH METHODS AND PROCEDURES

Subcutaneous (SQ) and omental adipose (OM) tissue taken during surgery from 12 morbidly obese subjects (10 women and 2 men) were cultured for up to 24 hours with insulin (7 nM) and/or dexamethasone (25 nM), a synthetic glucocorticoid, in the presence or absence of isoproterenol (10 microM). Adipose tissue was also acutely incubated for 3 hours in media alone with or without isoproterenol. Leptin secretion and leptin mRNA abundance were measured.

RESULTS

Iso acutely decreased leptin release by approximately 30% (vs. no hormone controls) in fragments of OM and SQ adipose tissue. In 24-hour culture, addition of Iso (in the presence of insulin) resulted in lower leptin accumulation in the medium (-20-30%) and leptin mRNA levels (-40-50%) from both tissue depots. Culture with insulin and dexamethasone increased leptin expression vs. insulin alone. Addition of Iso with insulin and dexamethasone decreased media leptin (-40-60%) and leptin mRNA levels were lower (-65%) in Iso-treated adipose tissue from both depots after 24 hours. Iso effects were not detectable after 5 hours of culture.

DISCUSSION

We conclude that stimulation of beta-adrenergic receptors may modulate leptin expression in human adipose tissue by two mechanisms: an acute effect on leptin release and a longer-term antagonism of stimulatory effects of insulin and dexamethasone on leptin mRNA expression. These mechanisms may contribute to the decline in serum leptin that occurs during fasting.

A role for leptin in hemopoieses?

The recent discovery of leptin as a major controller of appetite has led to a detailed analysis of its specific actions in this process as well as any potential role in the etiology of obesity. It has also emerged that leptin has a wider spectrum of biological activities and has been strongly implicated in fertility and reproduction. The structural similarity between leptin and its receptor and cytokine-receptor systems that control hemopoiesis has also prompted investigation of the potential for this hormone to influence blood cell formation. Recent studies have shown that the leptin receptor is expressed on a diverse range of hemopoietic cells. Leptin itself appears to enhance proliferation of hemopoietic cells in vitro, particularly in combination with other cytokines and may augment some mature hemopoietic cell functions. Although only relatively minor hemopoietic deficiencies have been reported in mice lacking leptin or its receptor, these emerging studies suggest that further analysis of leptin actions in hemopoiesis may be warranted.

Physiologic and endocrinologic characterization of male sex-biased diabetes in C57BLKS/J mice congenic for the fat mutation at the carboxypeptidease E locus

The fat gene in mice represents a recessive mutation at the carboxypeptidase E (Cpe) locus. The mutant allele (Cpe(fat)) encodes a highly unstable enzyme and produces an obesity phenotype characterized by attenuated processing of prohormones such as proinsulin that require this exopeptidase for full maturation. This article presents a preliminary physiologic and endocrinologic characterization of the stock of C57BLKS/LtJ-Cpe(fat)/Cpe(fat) mice at the backcross generation (N10) currently distributed by The Jackson Laboratory. Although previously reported not to be diabetogenic at N5, an additional five backcrosses to the C57BLKS/J background resulted in a male-biased development of both obesity and diabetes. Major differences distinguishing this mutant stock from the phenotypes produced by either the diabetes (Lepr(db)) or obese (Lep(ob)) mutations on the same inbred strain background are lack of hyperphagia and hypercorticism, sensitivity of diabetic males to exogenous insulin, and a milder and male-biased diabetes syndrome that is not associated with widespread beta-cell necrosis and islet atrophy, and that often remits with age.

No effect of Trp64Arg beta3-adrenoceptor polymorphism on the plasma leptin concentration in Pima Indians

In rodents, administration of leptin promotes beta3-adrenergic stimulation of thermogenesis in brown adipose tissue. Conversely, administration of a beta3-adrenoceptor (beta3-AR) agonist decreases leptin mRNA expression and secretion, suggesting that leptin and sympathetic nervous system activity mediated through the beta3-AR comprise a negative-feedback loop. It has recently been proposed that a defect in the beta3-AR in humans may contribute to a resistance to the sympathetically mediated effects of leptin on thermogenesis and lipolysis, thus leading to obesity and type 2 diabetes mellitus. We thus hypothesized that the Trp64Arg variant in the human beta3-AR would be associated with elevated plasma leptin concentrations. We studied 101 healthy nondiabetic Pima Indians: 11 Arg64 homozygotes, 35 Trp64 homozygotes, and 55 heterozygotes. The fasting plasma leptin concentration as an absolute value or after adjustment for percent body fat and sex was not associated with the beta3-AR genotype. Thus, the data do not support an influence of the Trp64Arg variant on the plasma leptin concentration.

Differential effects of fat and sucrose on body composition in A/J and C57BL/6 mice

The C57BL/6 (B6) mouse is more sensitive to the effects of a high-fat diet than the A/J strain. The B6 mouse develops severe obesity, hyperglycemia, and hyperinsulinemia when fed this dietary regimen. This study was conducted to determine the effects of dietary fat and sucrose concentrations on body composition and intestinal sucrase (EC 3.2.1.48) and maltase (EC 3.2.1.20) activity in these two mouse strains. High-fat diets, regardless of sucrose content, resulted in significant weight gain, higher body fat, and lower body protein and water content in both strains of mice. The shift toward higher body fat and lower protein and water content was far greater in the B6 strain. Low-fat, high-sucrose diets resulted in lower body weight in both strains, as well as significantly greater body protein content in B6 mice. Analysis of intestinal sucrase showed that the enzyme was less active in B6 mice when the diet was high in sucrose. Both sucrase and maltase had lower activity in the presence of high dietary fat in both mouse strains. The percent reduction of intestinal enzyme activity due to dietary fat was similar in both strains. The B6 mouse exhibits disproportionate weight gain and altered body composition on a high-fat diet. This coupled with the reduced body weight and increased body protein on a low-fat, high-sucrose diet suggests that factors-relative to fat metabolism rather than sucrose metabolism are responsible for obesity.

Circulating leptin did not associate with the development of the hyperglycemia accompanied by insulin insensitivity in spontaneous noninsulin dependent diabetes mellitus model Otsuka-Long-Evans-Tokushima-Fatty rats

Leptin, the product of the ob gene, has been reported to regulate feeding behavior and energy metabolism. Plasma leptin concentration was strongly correlated with body fat content in humans. It is well known that increased body fat content is accompanied by insulin insensitivity. In order to study the relationship between serum leptin level and metabolic variables, we performed caloric restriction on Otsuka-Long-Evans-Tokushima-Fatty (OLETF) rats, a model of noninsulin dependent diabetes mellitus. The male OLETF rats were allocated at random to three groups: 100% group, and 85% and 70% groups (which consumed 85% and 70% of the amount of food consumed by the 100% group, respectively). A significant correlation between serum leptin level and the body fat content, body weight, triglyceride, and fasting plasma glucose was observed. Using a partial correlation analysis to control for body fat content, however, the correlation between serum leptin and these variables disappeared. No significant changes in serum leptin levels were observed before and after a 1 h hyperinsulinemic euglycemic clamp test. In conclusion, serum leptin was significantly correlated with body fat content rather than fasting plasma glucose, serum insulin and insulin sensitivity. This suggests that circulating leptin per se may not result in hyperinsulinemia and insulin insensitivity in the OLETF rat.

Leptin: physiology and pathophysiology

The identification and sequencing of the ob gene and its product, leptin, in late 1994 opened new insights in the study of the mechanisms controlling body weight and led to a surge of research activity. During this time, a considerable body of knowledge regarding leptin's actions has been accumulated and the field continues to expand rapidly. Currently there is particular interest in the interaction of leptin with other peripheral and neural mechanisms to regulate body weight, reproduction and immunological response. In this review, we attempt to place the current state of knowledge about leptin in the broader perspective of physiology, including its structural characteristics, receptors, binding proteins, signalling pathways, regulation of adipose tissue expression and production, secretion patterns, clearance mechanisms and functional effects. In addition, leptin's involvement in the pathophysiology of obesity, anorexia nervosa, diabetes mellitus, polycystic ovary syndrome, acquired immunodeficiency syndrome, cancer, nephropathy, thyroid disease, Cushing's syndrome and growth hormone deficiency will be reviewed.

Clinical aspects of leptin

Hyperleptinemia is an essential feature of human obesity. Total body fat mass > % body fat > BMI are the best predictors of circulating leptin levels. Although ob gene is differentially expressed in different fat compartments, apart from total body fat, upper or lower body adiposity or visceral fat does not influence basal leptin levels. Similarly, age, basal glucose levels, and ethnicity do not influence circulating leptin levels. Only in insulin-sensitive individuals do basal levels of insulin and leptin correlate positively even after factoring in body fat. Diabetes does not influence leptin secretion in both lean and obese subjects per se. Independent of adiposity, leptin levels are higher in women than in men. This sexual dimorphism is also present in adolescent children. In eating disorders anorexia nervosa and bulimea nervosa, leptin levels are not upregulated but simply reflect BMI and probably body fat. In spite of strong correlation between body fat and leptin levels, there is great heterogeneity in leptin levels at any given index of body fat. About 5% of obese populations can be regarded as "relatively" leptin deficient which could benefit from leptin therapy. Leptin has dual regulation in human physiology. During the periods of weight maintenance, when energy intake and energy output are equal, leptin levels reflect total bodyfat mass. However, in conditions of negative (weight-loss programs) and positive (weight-gain programs) energy balances, the changes in leptin levels function as a sensor of energy imbalance. This latter phenomenon is best illustrated by short-term fasting and overfeeding experiments. Within 24 h of fasting leptin levels decline to approximately 30% of initial basal values. Massive overfeeding over a 12-h period increases leptin levels by approximately 50% of initial basal values. Meal ingestion does not acutely regulate serum leptin levels. A few studies have shown a modest increase in leptin secretion at supraphysiological insulin concentrations 4-6 h following insulin infusion. Under in vitro conditions, insulin stimulates leptin production only after four days in primary cultures of human adipocytes, which is apparently due to its trophic effects and an increased fat-cell size. Similar to other hormones, leptin secretion shows circadian rhythm and oscillatory pattern. The nocturnal rise of leptin secretion is entrained to mealtime probably due to cumulative hyperinsulinemia of the entire day. Like other growth factors and cytokines, leptin binding proteins including soluble leptin receptor are present in human serum. In lean subjects, the majority of leptin circulates in the bound form whereas in obese subjects, the majority of leptin is present in the free form. When free-leptin levels are compared between lean and obese subjects, even more pronounced hyperleptinemia in obesity is observed than that reported by measuring total leptin levels. During short-term fasting, free-leptin levels in lean subjects decrease in much greater proportion than those in obese subjects. In lean subjects with a relatively small energy store and particularly during food deprivation, leptin circulating predominantly in the bound form could be the mechanism to restrict its availability to hypothalamic leptin receptors for inhibiting leptin's effect on food intake and/or energy metabolism. Unlike marked changes in serum leptin, CSF leptin is only modestly increased in obese subjects and the CSF leptin/serum leptin ratio decreases logarithmically with increasing BMI. If CSF leptin levels are any indication of brain interstitial fluid levels, then hypothalami of obese subjects are not exposed to abnormally elevated leptin concentrations. In the presence of normal leptin receptor (functional long form, i.e., OB-Rb) mRNA expression and in the absence of leptin receptor gene mutations, it is logical to assume defective leptin signaling and/or impaired affector system(s) are the likely causes of leptin resistance in

Effect of bile acid composition and manipulation of enterohepatic circulation on leptin gene regulation

In the rat, the ob gene product, leptin, putatively regulates energy balance via appetite control and energy expenditure. Bile acids in the intestinal lumen are necessary for efficient absorption of dietary lipids and may trigger the release of regulatory peptides. To investigate whether bile acids play a role in leptin gene expression, we altered the bile acid pool and then measured leptin mRNA levels in adipose tissue. Rats fed cholic acid (1% of chow wt/wt) for 2 weeks did not gain weight as rapidly as pair-fed control animals. Despite the lower weight, normalized leptin mRNA levels were 24% greater in cholic acid-fed rats compared with controls. Conversely, cholestyramine, a bile acid sequestrant, in chow (5% wt/wt) resulted in a 26% decline in leptin mRNA. Ligation of the common bile duct or chronic biliary diversion, experimental manipulations that decreased the intestinal concentration of bile salts, decreased leptin gene expression by 30% and 50%, respectively. A fluid and electrolyte (F/E) solution with and without taurocholate (36 micromol/h x 100 g rat[-1]) was then infused for 12 hours into the duodenum in animals with chronic biliary diversion. Taurocholate infusion resulted in a fourfold increase in steady-state adipocyte leptin mRNA levels compared with F/E infusion. Intravenous infusion of taurocholate or incubation of cultured adipocytes with taurocholate had no effect on leptin mRNA levels. We conclude that bile acids upregulate leptin gene expression indirectly, probably via effects on the absorption of dietary lipids or the release of neurohumoral mediators.