Positional cloning of the mouse obese gene and its human homologue

Down-regulation of the expression of the obese gene by an antidiabetic thiazolidinedione in Zucker diabetic fatty rats and db/db mice

Obese (ob) is a recently identified gene involved in the regulation of energy balance in the mouse. We report here that AD-5075, a potent thiazolidinedione which lowered plasma glucose and triglyceride in Zucker diabetic fatty (ZDF) rats and db/db mice, decreased the expression of the ob gene in these animal models of obesity and non-insulin-dependent diabetes mellitus. The level of adipose ob mRNA in ZDF rats was 3-fold greater than that detected in the Zucker lean littermates. Chronic treatment with AD-5075 elicited a 67 and 70% reduction of ob mRNA in ZDF and control lean rats, respectively. Furthermore, the amount of adipose ob mRNA in db/db mice was 7 times higher than that detected in lean littermates. Treatment of db/db mice with AD-5075 resulted in a 78% reduction of the level of ob mRNA with parallel changes in circulating level of the ob gene product, leptin. The reduction of the ob mRNA in the Zucker lean rats was accompanied by significantly greater food intake and weight gain. However, in ZDF rats and db/db mice, there was profound increase in body weight without hyperphagia. The results demonstrate that the expression of the ob gene is up-regulated in these two rodent models of diabetes compared to their lean counterparts and that such overexpression is attenuated by treatment with an agent that improves insulin sensitivity and glucose homeostasis in vivo.

Identification and characterization of the mouse obesity gene tubby: a member of a novel gene family

The mutated gene responsible for the tubby obesity phenotype has been identified by positional cloning. A single base change within a splice donor site results in the incorrect retention of a single intron in the mature tub mRNA transcript. The consequence of this mutation is the substitution of the carboxy-terminal 44 amino acids with 24 intron-encoded amino acids. The normal transcript appears to be abundantly expressed in the hypothalamus, a region of the brain involved in body weight regulation. Variation in the relative abundance of alternative splice products is observed between inbred mouse strains and appears to correlate with an intron length polymorphism. This allele of tub is a candidate for a previously reported diet-induced obesity quantitative trait locus on mouse chromosome 7.

Pharmacologic manipulation of ob expression in a dietary model of obesity

Mutation of the obese (ob) gene results in severe hereditary obesity and diabetes in the C57BL/6J and related strains of mice. In this study we examined the expression of the ob gene in a dietary model in which moderate obesity develops in response to fat (58% of calories from fat) without mutation of the ob gene, and in four genetic models of obesity in mice: ob/ob, db/db, tubby, and fat. Several white and brown adipose depots were examined (epididymal, subcutaneous, perirenal, and interscapular). Northern blot analysis shows that levels of ob mRNA are increased in all adipose depots examined in every model of obesity. The average fold increases were 12.0 +/ 2.1 (ob/ob), 4.8 +/- 1.5 (db/db), 2.8 +/- 0.1 (tubby), 2.4 +/- 0.3 (fat), and 2.1 +/- 0.2 (high fat diet-induced A/J). Moreover, we found that the expression of the ob gene could be manipulated by pharmacologically blocking the development of diet-induced obesity. Supplementation of a high fat diet with a beta 3-adrenergic receptor agonist (CL316,243) prevented obesity, but not hyperphagia associated with high fat feeding (body weights of high fat-fed A/J mice = 34.0 +/- 1.0 g; high fat plus CL316,243-fed mice = 26.8 +/- 0.5 g; n = 10). CL316,243-treated, high fat-fed animals contained levels of ob mRNA in all adipose depots that were equal to or less than levels in low fat-fed mice (average levels in high fat plus CL316,243-fed mice relative to low fat-fed mice: 0.93 +/- 0.09). Inasmuch as fat cell size, but not number, was increased in a previous study in diet-induced obese A/J mice, these results indicate that expression of the ob gene serves as a sensor of fat cell hypertrophy, independent of any effects on food intake.

Obese gene expression: reduction by fasting and stimulation by insulin and glucose in lean mice, and persistent elevation in acquired (diet-induced) and genetic (yellow agouti) obesity

Mutations in the obese (ob) gene lead to obesity. This gene has been recently cloned, but the factors regulating its expression have not been elucidated. To address the regulation of the ob gene with regard to body weight and nutritional factors, Northern blot analysis was used to assess ob mRNA in adipose tissue from mice [lean, obese due to diet, or genetically (yellow agouti) obese] under different nutritional conditions. ob mRNA was elevated in both forms of obesity, compared to lean controls, correlated with elevations in plasma insulin and body weight, but not plasma glucose. In lean C57BL/6J mice, but not in mice with diet-induced obesity, ob mRNA decreased after a 48-hr fast. Similarly, in lean C57BL/6J controls, but not in obese yellow mice, i.p. glucose injection significantly increased ob mRNA. For up to 30 min after glucose injection, ob mRNA in lean mice significantly correlated with plasma glucose, but not with plasma insulin. In a separate study with only lean mice, ob mRNA was inhibited >90% by fasting, and elevated approximately 2-fold 30 min after i.p. injection of either glucose or insulin. These results suggest that in lean animals glucose and insulin enhance ob gene expression. In contrast to our results in lean mice, in obese animals ob mRNA is elevated and relatively insensitive to nutritional state, possibly due to chronic exposure to elevated plasma insulin and/or glucose.

YAC clone contigs covering 5 Mb of a repeat sequence island on the mouse X chromosome

We have initiated work towards the construction of YAC clone contigs across a repeat sequence island region on the mouse X Chromosome (Chr). The repeat sequence island region-the 141 island-located at band A3 contains 50 copies of a localized long complex repeat unit (LCRU). We have isolated 87 YAC clones from the 141 island and have used a dual faceted approach towards the construction of contigs across the repeat sequence island. First, we have identified YAC clones originating from the same region of the island by the identification of commonly held LCRU restriction site variants. Second, we have constructed rare cutter restriction maps of each YAC clone. Taken together, we have been able to assemble one large contig of 2.8 Mb and a number of smaller contigs. In total, contigs covering 5Mb of the island region have been identified. The island region would appear to represent a major component of the A3 Giemsa dark band on the mouse X Chr.

A role for melanin-concentrating hormone in the central regulation of feeding behaviour

The hypothalamus plays a central role in the integrated regulation of energy homeostasis and body weight, and a number of hypothalamic neuropeptides, such as neuropeptide Y (ref. 1), galanin, CRH (ref. 3) and GLP-1 (ref. 4), have been implicated in the mediation of these effects. To discover new hypothalmic peptides involved in the regulation of body weight, we used differential display polymerase chain reaction to identify messenger RNAs that are differentially expressed in the hypothalamus of ob/+ compared with ob/ob C57B1/6J mice. We show here that one mRNA that is overexpressed in the hypothalamus of ob/ob mice encodes the neuropeptide melanin-concentrating hormone (MCH). Fasting further increased expression of MCH mRNA in both normal and obese animals. Neurons containing MCH are located in the zona incerta and in the lateral hypothalamus. These areas are involved in regulation of ingestive behaviour, but the role of MCH in mammalian physiology is unknown. To determine whether MCH is involved in the regulation of feeding, we injected MCH into the lateral ventricles of rats and found that their food consumption increased. These findings suggest that MCH participates in the hypothalamic regulation of body weight.

Regulation of expression of ob mRNA and protein by glucocorticoids and cAMP

Regulation of obese gene (ob) expression in ob/ob and db/db mice and in cultured rat adipocytes was examined. It has been demonstrated that exogenous human OB protein (leptin) treatment reduces food intake and weight gain, as well as insulin, glucose, and corticosterone levels in ob/ob mice. In the present report we show that leptin treatment down-regulates endogenous adipose ob mRNA. However, treatment of isolated rat adipocytes with 100 ng/ml human or murine leptin had no direct effect on expression of endogenous ob mRNA, suggesting that leptin may be able to down-regulate its own expression by an indirect, non-autocrine mechanism. Glucocorticoids increased both ob mRNA levels and secreted leptin levels in vitro. Conversely, agents that increase intracellular cAMP, such as beta-adrenergic agonists or Bt2cAMP itself, decreased ob mRNA expression and leptin secretion. Therefore, increased glucocorticoid levels and decreased sympathetic neural activity may contribute to the elevated ob mRNA expression observed in genetically obese, hyperglucocorticoid rodents. Furthermore, leptin might regulate its own expression through a feedback mechanism involving the hypothalamic pituitary axis.

Abnormal regulation of hepatic glucocorticoid receptor mRNA and receptor protein distribution in the obese Zucker rat

This study examines the cellular distribution of glucocorticoid receptor (GR) protein and transcriptional activity of the GR gene in the liver of Zucker obese (fa/fa) rats. Immunoabsorption and Western blotting showed an increase in nuclear GR protein level but a decrease in cytosolic GR levels in the liver of 5-week old male obese rats (fa/fa) compared to their lean littermates (Fa/-). These changes were confirmed by receptor-ligand binding assays with [3H]-dexamethasone which showed a sixfold increase in average obese nuclear GR binding and a twofold reduction in cytosolic GR binding. HSP90, but not HSP70, levels were reduced in hepatic cytosol and increased in hepatic nuclei prepared from obese rats. Using Northern blot analysis of hepatic RNA, we demonstrated a twofold increase in hepatic mRNAs for GR, malic enzyme (ME), tyrosine aminotransferase (TAT), and glyceraldehyde 3-PO4-dehydrogenase in the obese rat. Increased transcription of GR and ME mRNAs in obese nuclei was indicated in nuclear run-on assays. These data suggest that there is increased nuclear localization of GR in the liver of obese rats and suggests that increased transcription of GR gene may contribute to this effect. The described changes may contribute to the abnormal regulation by glucocorticoids of some hepatic genes in the Zucker fa/fa rat.

Correction of the sterility defect in homozygous obese female mice by treatment with the human recombinant leptin

The sterility of male and female homozygous ob/ob mice is a recognized feature of the ob mutation (1). Whereas ob/ob males can occasionally reproduce if maintained on a restricted diet, ob/ob females are always sterile (2). Thinning of the ob/ob females to normal weight by diet-restriction failed to correct their sterility. Early sexual development is normal in ob/ob females; however, ovulation never follows and the mice remain prepuberal indefinitely with no occurrence of oestrus cycles. Reproductive hormones are reduced in ob/ob females (3) demonstrating a functional defect from the hypothalamic-pituitary axis (4-6). The ovaries of ob/ob females are capable of producing viable eggs when transplanted into lean female recipients (7). Reconstitution of reproductive functions in the ob/ob female necessitates delivery of hypothalamic extracts to the third ventricle (8) and administration of pituitary extract (9), gonadotropic hormones (10), progesterone (11) and relaxin (12). These previous findings demonstrate that the sterility of ob/ob females is caused by an insufficiency of hormones at the hypothalamic-pituitary level rather than physical hindrance of copulatory activity, pregnancy and parturition caused by excess adipose tissue. We show here that repeated administration of only the recombinant human ob protein, leptin, into homozygous female ob/ob mice can correct their sterility, thus resulting in ovulation, pregnancy and parturition.

The ontogeny of resource allocation in giant transgenic rat growth hormone mice

Dry mass budgets were conducted on transgenic metallothionein-1 rat growth hormone mice and normal Mus musculus to assess ontogenic changes in growth, feeding, and resource allocation. Younger mice had higher rates and efficiencies of growth than older mice. Young transgenic mice and normal controls were relatively similar for most features but became progressively dissimilar with time. The rate of growth of transgenics was never faster than the most rapid growth observed in normal mice, but they grew larger by maintaining a higher growth rate to a later age. On a mass-specfic basis, transgenic animals consumed less food than normal ones. Reduced feeding was not simply a reflection of the allometric scaling of food intake with larger body size, as younger transgenic mice ate less food than normal ones of equivalent size, even on an absolute basis. Transgenic mice achieved increased growth via superior production efficiency and ontogenically by maintaining greater efficiency to a later age. Differences in feeding and efficiency were detectable even before the mice diverged much in size. A single relationship relating production efficiencies and growth rates for older mice was confirmed, but younger transgenic mice and normal controls displayed fundamentally different relationships between efficiencies and rates of growth. Insights into growth regulation, feeding, life-history trade-offs, and allometric theory are discussed.

Nocturnal rise of leptin in lean, obese, and non-insulin-dependent diabetes mellitus subjects

We studied 24-h profiles of circulating leptin levels using a sensitive and specific RIA in lean controls and obese subjects with or without non-insulin-dependent diabetes mellitus (NIDDM) during normal routine activity. Serum leptin levels were significantly higher in obese (41.7 +/- 9.0 ng/ml; n = 11) and obese NIDDM (30.8 +/- 6.7; n = 9) subjects compared with those in lean controls (12.0 +/- 4.4, n = 6). In all the three groups, serum leptin levels were highest between midnight and early morning hours and lowest around noon to midafternoon. The nocturnal rise in leptin levels was significant when data were analyzed by ANOVA (lean: F = 3.17, P < 0.0001, n = 4; obese: F = 2.02, P < 0.005, n = 11; and obese NIDDM: F = 4.9, P < 0.0001, n = 5). The average circadian amplitude between acrophase and nadir was 75.6% in lean, 51.7%, in obese and 60.7% in obese NIDDM groups, respectively. No significant correlations (P > 0.05) were observed between circulating levels of leptin and either insulin or glucose levels in any of the 20 subjects studied for 24-h profiles. The nocturnal rise in leptin observed in the present study resembles those reported for prolactin, thyroid-stimulating hormone, and free fatty acids. We speculate that the nocturnal rise in leptin could have an effect in suppressing appetite during the night while sleeping.

Leptin: the weight-reducing plasma protein encoded by the obese gene

Leptin, the protein encoded by the recently cloned obese gene, has the properties of a hormone released by adipose tissue, regulating appetite and energy expenditure. Injected leptin reduces body weight and food intake in mice, and in obese, diabetic mice (with a mutated obese gene), it also reduces plasma insulin and glucose. Leptin release is stimulated by insulin; leptin appears to act on the hypothalamus by inhibiting the release of the neuropeptide Y.

Intracerebroventricular administration of neuropeptide Y to normal rats increases obese gene expression in white adipose tissue

The aim of this work was to determine the possible inter-relationship between neuropeptide Y (NPY, a hypothalamic stimulator of feeding) and adipose tissue expression of the ob protein (a novel potent inhibitor of feeding). Such a relationship could be of importance in the maintenance of normal body weight. To this end, normal rats were intracerebroventricularly (i.c.v.) infused for 6 days with NPY. NPY infusion resulted in hyperphagia and a marked increase in adipose tissue ob mRNA levels. The effect of NPY on ob expression persisted when hyperphagia was prevented by pair-feeding, and was reversed following cessation of NPY infusion. Basal and glucose-stimulated insulinaemia were increased by i.c.v. NPY infusion compared to control values, regardless of whether animals were ad libitum-fed or pair-fed. Cessation of NPY infusion was accompanied by normalisation of insulinaemia. These changes in insulinaemia produced by i.c.v. NPY infusion paralleled the observed changes in ob expression. When normal rats were made hyperinsulinaemic-euglycaemic for 24 h, such hyperinsulinaemia also resulted in increased ob mRNA levels in white adipose tissue. This suggested that NPY-induced hyperinsulinaemia could be responsible for the upregulation of ob mRNA levels of NPY-infused rats. It is concluded that central (i.c.v.) NPY infusion increases adipose tissue ob expression, a functional relationship that is linked, at least in part, via NPY-induced hyperinsulinaemia.

Assortative mating for relative weight: genetic implications

Most work on the genetics of relative weight has not considered the role of assortative mating, i.e., mate selection based on similarity between mates. We investigated the extent to which engaged men and women in an archival longitudinal database were similar to each other in relative body weight prior to marriage and cohabitation. After controlling for age, a small but statistically significant mate correlation was found for relative weight (r=.13, p=.023), indicating some assortative mating. Furthermore, we examined whether mate similarity in relative weight prior to marriage predicts survival of the marriage. No significant effects were found. In sum, these results are consistent with those of other studies in suggesting that there is a small but significant intermate correlation for relative weight. However, they are unique in showing that these results cannot be explained on the basis of (a) cohabitation, (b) age similarity, or (c) selective survival of marriages between couples more similar in relative weight. The implications of these findings for heritability studies, linkage studies, and the estimation of shared environmental effects are discussed.

Genomic structure and promoter analysis of the human obese gene

The human gene encoding the homolog of the mouse obese (ob) gene was isolated and partially characterized. The human ob gene consists of three exons and two introns and spans about 18 kilobase pairs (kb), encoding a 3.5-kb cDNA. A 3-kb 5'-flanking region of the gene was cloned and transient transfection assay with luciferase reporter confirmed the promoter activity in differentiated F442-A adipocytes. Potential regulatory elements are discussed in this report.

Decreased food intake does not completely account for adiposity reduction after ob protein infusion

The effects of recombinantly produced ob protein were compared to those of food restriction in normal lean and genetically obese mice. Ob protein infusion into ob/ob mice resulted in large decreases in body and fat-depot weight and food intake that persisted throughout the study. Smaller decreases in body and fat-depot weights were observed in vehicle-treated ob/ob mice that were fed the same amount of food as that consumed by ob protein-treated ob/ob mice (pair feeding). In lean mice, ob protein infusion significantly decreased body and fat-depot weights, while decreasing food intake to a much lesser extent than in ob/ob mice. Pair feeding of lean vehicle-treated mice to the intake of ob protein-treated mice did not reduce body fat-depot weights. The potent weight-, adipose-, and appetite-reducing effects exerted by the ob protein in ob protein-deficient mice (ob/ob) confirm hypotheses generated from early parabiotic studies that suggested the existence of a circulating satiety factor of adipose origin. Pair-feeding studies provide compelling evidence that the ob protein exerts adipose-reducing effects in excess of those induced by reductions in food intake.

Phenotypes of mouse diabetes and rat fatty due to mutations in the OB (leptin) receptor

Mice harboring mutations in the obese (ob) and diabetes (db) genes display similar phenotypes, and it has been proposed that these genes encode the ligand and receptor, respectively, for a physiologic pathway that regulates body weight. The cloning of ob, and the demonstration that it encodes a secreted protein (leptin) that binds specifically to a receptor (OB-R) in the brain, have validated critical aspects of this hypothesis. Here it is shown by genetic mapping and genomic analysis that mouse db, rat fatty (a homolog of db), and the gene encoding the OB-R are the same gene.

Abnormal splicing of the leptin receptor in diabetic mice

Mutations in the mouse diabetes (db) gene result in obesity and diabetes in a syndrome resembling morbid human obesity. Previous data suggest that the db gene encodes the receptor for the obese (ob) gene product, leptin. A leptin receptor was recently cloned from choroid plexus and shown to map to the same 6-cM interval on mouse chromosome 4 as db. This receptor maps to the same 300-kilobase interval as db, and has at least six alternatively spliced forms. One of these splice variants is expressed at a high level in the hypothalamus, and is abnormally spliced in C57BL/Ks db/db mice. The mutant protein is missing the cytoplasmic region, and is likely to be defective in signal transduction. This suggests that the weight-reducing effects of leptin may be mediated by signal transduction through a leptin receptor in the hypothalamus.

Evidence that the diabetes gene encodes the leptin receptor: identification of a mutation in the leptin receptor gene in db/db mice

OB-R is a high affinity receptor for leptin, an important circulating signal for the regulation of body weight. We identified an alternatively spliced transcript that encodes a form of mouse OB-R with a long intracellular domain. db/db mice also produce this alternatively spliced transcript, but with a 106 nt insertion that prematurely terminates the intracellular domain. We further identified G --> T point mutation in the genomic OB-R sequence in db/db mice. This mutation generates a donor splice site that converts the 106 nt region to a novel exon retained in the OB-R transcript. We predict that the long intracellular domain form of OB-R is crucial for initiating intracellular signal transduction, and as a corollary, the inability to produce this form of OB-R leads to the severe obese phenotype found in db/db mice.

Expression of ob gene in adipose cells. Regulation by insulin

The product of the recently cloned mouse obese (ob) gene is likely to play an important role in a loop regulating the size of the adipose tissue mass. The hormonal regulation of the ob gene could affect adiposity. To investigate this point, the effect of insulin on ob gene expression was examined in cells of the 3T3-F442A preadipocyte clonal line. ob mRNA is absent from exponentially growing, undifferentiated cells as well as from confluent preadipose cells. Terminal differentiation of preadipose to adipose cells leads to the expression of ob mRNA detected by a sensitive and quantitative ribonuclease protection assay. In adipose cells, the level of ob mRNA is sensitive to insulin in the nanomolar range of concentrations with an increase from an average of 1 copy to 5-10 copies/cell. The effect of insulin was fully reversible and takes place primarily at a transcriptional level. The ob mRNA shows a rapid turnover, with a half-life of approximately 2 h in the absence or presence of insulin. The level of secreted Ob protein is also regulated by insulin. These results indicate that the ob gene is expressed in mature fat cells only and support the possibility that insulin is an important regulator of ob gene expression.

Sub-milliMorgan map of the proximal part of mouse Chromosome 17 including the hybrid sterility 1 gene

We have generated a high-resolution genetic map, 0.071 cM per backcross animal, of the 13 cM T-H2 region of the mouse Chromosome (Chr) 17. The map contains two phenotypic loci, T and Hst1, 12 RFLP markers, and 24 microsatellite loci. The Hst1 gene was mapped to a chromosomal interval contained within a single 580-kb YAC clone. The FFEH11 YAC is 0.44 cM long and carries, besides the Hst1 gene, five polymorphic DNA markers and recombination breakpoints of six backcross animals. Two candidate genes for Hst1 were identified based on their location and testicular expression. These are Tbp and D17Ph4e. The submilliMorgan map of the T-H2 region revealed significant clustering of (CA)n loci. The clustering, if shown to be a common feature in the mouse genome, may cause gaps in the physical map of the mouse genome.

Current views on obesity

Although the disorders associated with obesity have been extensively studied, little attention has been paid to the fact that obesity is itself a chronic disease. This misunderstanding of the nature of obesity has contributed to the stigmatization of obese persons and to the use of inappropriate or inadequate treatment regimens. Although the etiology of obesity is still unclear, genetic, metabolic, and social factors are all believed to play a role in its development and progression. Behavioral therapy, exercise, very-low-calorie diets, drug therapy, and surgery affect the treatment of obesity of differing levels of severity. The regaining of weight following treatments other than surgery is very frequent, in part because periods of weight loss are rarely followed by maintenance programs. An increasing awareness of the chronic, multifactorial nature of obesity will ideally lead to the development of new long-term treatment programs that are safe and effective. Such programs are urgently needed in light of new data that show that the prevalence of obesity is increasing in the United States, as much as 30% in the last decade.

The new obesity genes

Individual susceptibility to obesity is recognized to be influenced significantly by genetic inheritance. Recently, candidate obesity genes have been identified that may contribute to the inheritance of body fat mass and the partitioning of fat between central and peripheral fat depots. In studies of animal models of obesity, the genetic basis for obesity in the obese (Ob/Ob) mouse, the Fat mouse, and the Yellow (Vvy) mouse has been identified. Further research is needed to determine whether abnormalities in these genes contribute to human obesity as well. In studies of humans, sequence variation in at least six genes has been linked to increased body fatness and/or susceptibility to obesity. In addition, five other encoding genes have been linked to a disproportionate storage of fat in the abdominal region. These genes identified in studies of humans are currently thought to be modifying or background genes, each separately conferring a modest increase in susceptibility to fatness. Further research is needed to identify additional candidate genes that confer susceptibility to obesity and to determine the relative importance of each one in a range of human populations with distinct environments.

Genes and obesity

Several twin and adoption studies as well as animal models have confirmed that obesity has a strong genetic component. It is apparent that obesity in humans has a complex polygenic background; that is, the phenotype is determined by an unknown number of genes together with environmental factors. However, there may well be single genes playing a major role within certain families, but the gene may vary from family to family. Two interesting gene candidates have been found in mouse experiments, the ob and the db gene, the products of which are probably a satiety hormone and its receptor, respectively, which regulate food intake. The recent cloning of the ob gene and its human homologue constitute a major breakthrough in this field. The 16-kD protein encoded by the ob gene is called leptin, and is well conserved among vertebrate species. The pig could be a valuable large animal model for human obesity. A fatness locus has been mapped to pig chromosome 4 and most probably a similar locus resides on human chromosome 1q. A more precise definition of the pig-human homology as regards this region is currently being investigated.

Serum immunoreactive-leptin concentrations in normal-weight and obese humans

BACKGROUND

Leptin, the product of the ob gene, is a hormone secreted by adipocytes. Animals with mutations in the ob gene are obese and lose weight when given leptin, but little is known about the physiologic actions of leptin in humans.

METHODS

Using a newly developed radioimmunoassay, wer measured serum concentrations of leptin in 136 normal-weight subjects and 139 obese subjects (body-mass index, > or = 27.3 for men and > or = 27.8 for women; the body-mass index was defined as the weight in kilograms divided by the square of the height in meters). The measurements were repeated in seven obese subjects after weight loss and during maintenance of the lower weight. The ob messenger RNA (mRNA) content of adipocytes was determined in 27 normal-weight and 27 obese subjects.

RESULTS

The mean (+/- SD) serum leptin concentrations were 31.3 +/- 24.1 ng per milliliter in the obese subjects and 7.5 +/- 9.3 ng per milliliter in the normal-weight subjects (P < 0.001). There was a strong positive correlation between serum leptin concentrations and the percentage of body fat (r = 0.85, P < 0.001). The ob mRNA content of adipocytes was about twice as high in the obese subjects as in the normal-weight subjects (P < 0.001) and was correlated with the percentage of body fat (r = 0.68, P < 0.001) in the 54 subjects in whom it was measured. In the seven obese subjects studied after weight loss, both serum leptin concentrations and ob mRNA content of adipocytes declined, but these measures increased again during the maintenance of the lower weight.

CONCLUSIONS

Serum leptin concentrations are correlated with the percentage of body fat, suggesting that most obese persons are insensitive to endogenous leptin production.

Transcriptional activation of the mouse obese (ob) gene by CCAAT/enhancer binding protein alpha

Like other adipocyte genes that are transcriptionally activated by CCAAT/enhancer binding protein alpha (C/EBP alpha) during preadipocyte differentiation, expression of the mouse obese (ob) gene is immediately preceded by the expression of C/EBP alpha. While the 5' flanking region of the mouse ob gene contains several consensus C/EBP binding sites, only one of these sites appears to be functional. DNase I cleavage inhibition patterns (footprinting) of the ob gene promoter revealed that recombinant C/EBP alpha, as well as a nuclear factor present in fully differentiated 3T3-L1 adipocytes, but present at a much lower level in preadipocytes, protects the same region between nucleotides -58 and -42 relative to the transcriptional start site. Electrophoretic mobility-shift analysis using nuclear extracts from adipose tissue or 3T3-L1 adipocytes and an oligonucleotide probe corresponding to a consensus C/EBP binding site at nucleotides -55 to -47 generated a specific protein-oligonucleotide complex that was supershifted by antibody against C/EBP alpha. Probes corresponding to two upstream consensus C/EBP binding sites failed to generate protein-oligonucleotide complexes. Cotransfection of a C/EBP alpha expression vector into 3T3-L1 cells with a series of 5' truncated ob gene promoter constructs activated reporter gene expression with all constructs containing the proximal C/EBP binding site (nucleotides -55 to -47). Mutation of this site blocked transactivation by C/EBP alpha. Taken together, these findings implicate C/EBP alpha as a transcriptional activator of the ob gene promoter and identify the functional C/EBP binding site in the promoter.

Regulation of ob gene mRNA levels in cultured adipocytes

mRNA levels of the ob gene product, leptin, were investigated by quantitative competitive RT-PCR in a mouse cell line (3T3-L1) which can be induced to differentiate into adipocytes. During conversion to fat cells, the level of leptin mRNA increased several-fold and in parallel to that for typical adipocyte markers like lipoprotein lipase, adipsin and glycerophosphate dehydrogenase. Leptin transcription, however, did not correlate with the size of the adipocytes measured as total triglycerides. On the other hand, mRNA levels for leptin in fully differentiated adipocytes were increased 2-3 fold by insulin. In contrast, free fatty acids exerted a concentration-dependent inhibition of leptin transcription while the corticosteroid dexamethasone and an elevation of intracellular cAMP displayed only marginal inhibitory effects on leptin mRNA levels.

Augmented expression of obese (ob) gene during the process of obesity in genetically obese-hyperglycemic Wistar fatty (fa/fa) rats

Expression of the obese (ob) gene is up-regulated in the adipose tissue in several obese rodent models. To study the regulation of the ob gene expression during the development of obesity, we examined the ob gene expression in genetically obese-hyperglycemic Wistar fatty (fa/fa) rats at several stages of obesity. The ob mRNA levels in the adipose tissue from Wistar fatty rats was unequivocally augmented and continued to rise in the process of obesity. Furthermore, the ob gene expression in this obese model was much more rapidly enhanced in the mesenteric fat than in the subcutaneous fat. Moreover, the ob gene expression was more greatly augmented in the mesenteric fat than the lipoprotein lipase gene expression. These results suggest the presence of obesity-linked and region-specific regulation of the ob gene expression.

Leptin enters the brain by a saturable system independent of insulin

Leptin, or OB protein, is produced by fat cells and may regulate body weight by acting on the brain. To reach the brain, circulating leptin must cross the blood-brain barrier (BBB). Intravenously injected radioiodinated leptin (125I-leptin) had an influx constant (Ki) into brain of (5.87)10(-4) ml/g-min, a rate 20 times greater than that of labeled albumin. Unlabeled leptin inhibited the influx of 125I-leptin in a dose-dependent manner whereas unlabeled tyrosine and insulin, which have saturable transport systems, were without effect. HPLC and acid precipitation showed that the radioactivity in brain and serum represented intact 125I-leptin. About 75% of the extravascular 125I-leptin in brain completely crossed the BBB to reach brain parenchyma. Autoradiography detected uptake at the choroid plexus, arcuate nuclei of the hypothalamus, and the median eminence. Saturable transport did not occur out of the brain. The results show that leptin is transported intact from blood to brain by a saturable system.

Lipoprotein profiles and glucose tolerance in lean and obese chimpanzees

We compared serum lipid profiles and glucose tolerance of obese and lean chimpanzees maintained on a 10.9% fat diet. Seven of 14 obese and 6 of 17 lean chimpanzees were hypercholesterolemic (low density lipoprotein cholesterol > 160 mg/dl), three obese and three lean animals had total cholesterol/high density lipoprotein cholesterol ratios of 5.9-10.7, and two obese and one lean chimpanzee had abnormal glucose tolerance. Useful numbers of captive chimpanzees thus exhibit metabolic abnormalities without recourse to high fat diets and could serve as surrogates in studies of human metabolic diseases.

Peripherally administered calcitonin gene-related peptide decreases food intake in mice

Previously, calcitonin gene-related peptide (CGRP) decreased food intake when administered ICV but not when administered peripherally to rats. Amylin, which has a close structural homology to CGRP, reduced food intake administered IP at concentrations higher than those previously tested for CGRP. We examined the effects of higher doses of IP-administered CGRP on food intake. CGRP reduced food intake from 25 to 200 micrograms/kg in mice. CGRP did not reduce water intake and was not aversive in a two-bottle test. Using a lever press, CGRP was more effective at reducing milk consumption in prefed than in nonprefed mice. The effect of CGRP on food intake was not antagonized by the cholecystokinin A receptor antagonist, L364,718. These studies suggest a role for CGRP as a satiating factor.

Upregulation of adipocyte metabolism by agouti protein: possible paracrine actions in yellow mouse obesity

Mutations leading to ectopic expression of the murine agouti gene (a) result in progressive obesity. To further characterize this model, we analyzed adipose and hepatic mRNA levels for fatty acid synthase (FAS) and stearoyl-CoA desaturase (SCD), two key enzymes in de novo fatty acid synthesis and desaturation, respectively. FAS and SCD mRNA in both tissues of obese (Avy) mice were dramatically increased relative to lean (ala) controls. Excessive expression of these genes in this model could be due to direct effects of the agouti gene product; to test this possibility we treated 3T3-L1 adipocytes in vitro with recombinant agouti protein. Agouti treatment increased FAS and SCD mRNA levels by 1.5- and 4-fold, respectively. In addition, FAS activity and triglyceride content were 3-fold higher in agoutitreated 3T3-L1 cells relative to controls; these effects were attenuated by simultaneous treatment with a calcium channel blocker (nitrendipine). These data demonstrate that the agouti protein can directly increase lipogenesis in adipocytes and suggest that these effects are mediated through an intracellular calcium-dependent mechanism.