Positional cloning of the mouse obese gene and its human homologue

Identification and expression cloning of a leptin receptor, OB-R

The ob gene product, leptin, is an important circulating signal for the regulation of body weight. To identify high affinity leptin-binding sites, we generated a series of leptin-alkaline phosphatase (AP) fusion proteins as well as [125I]leptin. After a binding survey of cell lines and tissues, we identified leptin-binding sites in the mouse choroid plexus. A cDNA expression library was prepared from mouse choroid plexus and screened with a leptin-AP fusion protein to identify a leptin receptor (OB-R). OB-R is a single membrane-spanning receptor most related to the gp130 signal-transducing component of the IL-6 receptor, the G-CSF receptor, and the LIF receptor. OB-R mRNA is expressed not only in choroid plexus, but also in several other tissues, including hypothalamus. Genetic mapping of the gene encoding OB-R shows that it is within the 5.1 cM interval of mouse chromosome 4 that contains the db locus.

Early alterations in the brown adipose tissue adenylate cyclase system of pre-obese Zucker rat fa/fa pups: decreased G-proteins and beta 3-adrenoceptor activities

This study was undertaken to determine whether receptor and non-receptor components of the adenylate cyclase (AC) cascade were altered in brown adipose tissue (BAT) of 14-day-old pre-obese (fa/fa) rats, before endocrine status is strongly modified by fa gene expression. Activity of the AC catalytic subunit did not differ between the two genotypes. In fa/fa rats compared with control Fa/fa rats, there was a 50% decrease in the activity of alpha Gs (stimulated by NaF or guanosine 5'-[gamma-thio]triphosphate) but no change in protein content (Western blotting). alpha Gi function, assessed by the inhibitory action of low concentrations of guanosine 5'-[beta gamma-imido]triphosphate upon 10(-4) M forskolin-stimulated AC activity, was equally low in both genotypes. Analysis of dose-response curves for different beta-agonists revealed that (i) both the basal and the maximally stimulated activity of AC were 2-fold lower in fa/fa rats than in Fa/fa rats; (ii) BRL37344 and CGP12177 (beta 3 agonists) were less potent in fa/fa than in Fa/fa rats (Kact. multiplied by 2); (iii) noradrenaline and isoprenaline (Iso), at the low-affinity site (beta 3-AR), were less potent in fa/fa than in Fa/fa pups (Kact. increased by 30 and 20% respectively). At the high-affinity site (mainly beta 1) these two agonists were more potent in fa/fa than in Fa/fa rats (Kact. decreased by 40 and 80% respectively). In good agreement with the latter result, the beta 1-adrenergic receptor (beta 1-AR)-selective antagonist CGP20712A had more effect on the Iso-stimulated AC activity in pre-obese than in lean pups (2-fold decreased in IC50). Binding experiments with [3H]CGP12177 show that in BAT of suckling rats, beta 3-ARs represent 80% of the total beta-ARs. Bmax values for the two sites were not affected by the genotype, although the beta 3-AR mRNA concentration in BAT (quantitative reverse-transcriptase PCR) was 3-fold lower in fa/fa rats than in Fa/fa pups. In conclusion, these results provide evidence for alterations in beta 1- and beta 3-AR signalling in BAT of 14-day-old suckling pre-obese Zucker rats with a decreased activity of alpha Gs. The impaired AC responsiveness to catecholamines might be a primary contributor to the development of this genetic obesity.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced anorexia and wasting syndrome in rats: aggravation after ventromedial hypothalamic lesion

Long-term regulation of body weight and food intake were studied after rats were subjected to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which causes hypophagia and body weight loss, and to ventromedial hypothalamic lesion, which causes hyperphagia, metabolic changes and obesity. These two factors appeared to have an interaction, as ventromedial hypothalamic lesion initially aggravated the effects of TCDD on body weight and food intake. This was seen in both TCDD-resistant and TCDD-susceptible rat strains. In contrast, if TCDD was given several weeks before the lesion and body weight had stabilized to a low level, no aggravation was seen, but TCDD completely blocked the effects of ventromedial hypothalamic lesion. Thus, TCDD seems to affect the same regulation chain that is involved in the lesioning of the ventromedial hypothalamus. TCDD might serve as a tool in studying different mechanisms of long-term food intake and body weight regulation.

Recombinant proteins for medical use: the attractions and challenges

Recombinant proteins that have survived the challenges of process development and clinical trials are becoming blockbuster medical products. Growth factors, enzymes and antibodies are being improved by mutational approaches, fused with other proteins, and even chemically modified in vitro. Drug development and testing approaches have advanced, and proteins produced in transgenic animals are new becoming available. Future protein products might include cancer vaccines and therapies for a variety of genetic diseases, but alternative treatments involving gene therapy or small synthetic compounds will provide competition.

Leptin levels reflect body lipid content in mice: evidence for diet-induced resistance to leptin action

The regulation of body weight and composition involves input from genes and the environment, demonstrated, for example, by the variable susceptibility of inbred strains of mice to obesity when offered a high-fat diet. The identification of the gene responsible for obesity in the ob/ob mouse provides a new approach to defining links between diet and genetics in the regulation of body weight. The ob gene protein product, leptin, is an adipocyte-derived circulating protein. Administration of recombinant leptin reduces food intake and increases energy expenditure in ob/ob mice, suggesting that it signals to the brain the magnitude of fat stores. Information on the regulation of this protein is limited. In several rodent models of obesity including db/db, fa/fa, yellow (Ay/a) VMH-lesioned, and those induced by gold thioglucose, monosodium glutamate, and transgenic ablation of brown adipose tissue, leptin mRNA expression and the level of circulating leptin are increased, suggesting resistance to one or more of its actions. We have assessed the impact of increased dietary fat on circulating leptin levels in normal FVB mice and FVB mice with transgene-induced ablation of brown adipose tissue. We find that high-fat diet evokes a sustained increase in circulating leptin in both normal and transgenic mice, with leptin levels accurately reflecting the amount of body lipid across a broad range of body fat. However, despite increased leptin levels, animals fed a high-fat diet became obese without decreasing their caloric intake, suggesting that a high content of dietary fat changes the 'set point' for body weight, at least in part by limiting the action of leptin.

The mouse obese gene. Genomic organization, promoter activity, and activation by CCAAT/enhancer-binding protein alpha

The obese gene product, leptin, regulates adiposity. Mice homozygous for a nonfunctional obese gene become massively obese and develop diabetes mellitus due to overeating and increased metabolic efficiency. The cDNA sequence of obese was recently reported (Zhang, Y., Proenca, R., Maffei, M., Barone, M., Leopold, L., and Friedman, J. L. (1994) Nature 372, 425-432; Correction: (1995 Nature 374, 479). We have determined the genomic organization of the 5' end of the mouse obese gene. The coding sequence is in exons 2 and 3. A single TATA-containing promoter was found upstream of exon 1. A minority (probably approximately 5%) of the obese mRNA contained an extra, untranslated exon between exons 1 and 2. Transcription of the obese gene was detected only in adipose cells. A 762-base pair obese gene promoter driving a luciferase gene yielded abundant activity in transiently transfected rat adipose cells in primary culture. The obese promoter was inactive in erythroid K562 cells. Deletion of bases from -762 downstream to -161 did not affect promoter activity in transfected adipose cells. The -161 minimal promoter contained consensus Sp1 and CCAAT/enhancer-binding protein (C/EBP) motifs. Cotransfection with C/EBP alpha (a transcription factor important in adipose cell differentiation) caused 23-fold activation. These data suggest that the obese promoter is a natural target of C/EBP alpha.

Regulation of adipose maturation and energy homeostasis

The adipose tissue of mammals represents a dynamic organ disseminated throughout the body. It fluctuates in abundance according to the availability of metabolic energy supplies. Mature adipose tissue communicates with the central nervous system via a hormonal circuit that controls satiety. Adipogenesis can be recapitulated in cell culture, thus facilitating molecular biological studies of the regulatory proteins that control this process. Such studies have led to the identification of two families of transcription factors that regulate adipogenesis and mammalian energy homeostasis.

Understanding the pathogenesis of type 2 diabetes: can we get off the metabolic merry-go-rounds?

The aetiology of non-insulin-dependent diabetes mellitus (NIDDM) is not known. The concordance of NIDDM in identical twins and differences in the prevalence rate of NIDDM between different racial groups suggest a genetic cause. Hyperglycaemia in established diabetes is caused by a combination of hepatic insulin resistance, impaired peripheral (muscle and fat) glucose uptake and a defect in glucose-mediated insulin secretion. However, it is not known if these defects are all inherited or if one can cause the others. This uncertainty is due to the fact that hyperglycaemia per se can cause defects in insulin action and insulin secretion that resemble those found in NIDDM. Furthermore the elevated free fatty acid (FFA) levels found when NIDDM is associated with obesity are known to cause both peripheral and hepatic insulin resistance. Recently we have demonstrated the mechanism by which elevated FFA levels can cause hepatic insulin resistance. However, we also have evidence that the converse holds in that genetically engineered hepatic insulin resistance in a transgenic rat model leads to obesity. Thus an understanding of the pathogenesis of NIDDM is complicated by the fact that hyperglycaemia and obesity can be both causes and consequences of insulin resistance. To overcome these difficulties, studies in young euglycaemic diabetes-prone subjects have been conducted. Results suggest that there may be different causes for NIDDM in different racial groups.

Mapping the mouse genome: current status and future prospects

The mouse is the best model system for the study of mammalian genetics and physiology. Because of the feasibility and importance of studying genetic crosses, the mouse genetic map has received tremendous attention in recent years. It currently contains over 14,000 genetically mapped markers, including 700 mutant loci, 3500 genes, and 6500 simple sequence length polymorphisms (SSLPs). The mutant loci and genes allow insights and correlations concerning physiology and development. The SSLPs provide highly polymorphic anchor points that allow inheritance to be traced in any cross and provide a scaffold for assembling physical maps. Adequate physical mapping resources--notably large-insert yeast artificial chromosome (YAC) libraries--are available to support positional cloning projects based on the genetic map, but a comprehensive physical map is still a few years away. Large-scale sequencing efforts have not yet begun in mouse, but comparative sequence analysis between mouse and human is likely to provide tremendous information about gene structure and regulation.

Structural organization and chromosomal assignment of the human obese gene

The obese (ob) gene has been identified through a positional cloning approach; the mutation of this gene causes marked hereditary obesity and diabetes mellitus in mice. We report here the isolation and characterization of the human ob gene. Southern blot analysis demonstrated a single copy of the ob gene in the human genome. The human ob gene spanned approximately 20 kilobases (kb) and contained three exons separated by two introns. The first intron, approximately 10.6 kb in size, occurred in the 5'-untranslated region, 29 base pair (bp) upstream of the ATG start codon. The second intron of 2.3 kb in size was located at glutamine +49. By rapid amplification of 5'-cDNA ends, the transcription initiation sites were mapped 54-57 bp upstream of the ATG start codon. The 172-bp 5'-flanking region of the human ob gene contained a TATA box-like sequence and several cis-acting regulatory elements (three copies of GC boxes, an AP-2-binding site, and a CCAAT/enhancer-binding protein-binding site). By the fluorescence in situ hybridization technique, the ob gene was assigned to human chromosome 7q31.3. This study should establish the genetic basis for ob gene research in humans, thereby leading to the better understanding of the molecular mechanisms underlying the ob gene.

A novel serum protein similar to C1q, produced exclusively in adipocytes

We describe a novel 30-kDa secretory protein, Acrp30 (adipocyte complement-related protein of 30 kDa), that is made exclusively in adipocytes and whose mRNA is induced over 100-fold during adipocyte differentiation. Acrp30 is structurally similar to complement factor C1q and to a hibernation-specific protein isolated from the plasma of Siberian chipmunks; it forms large homo-oligomers that undergo a series of post-translational modifications. Like adipsin, secretion of Acrp30 is enhanced by insulin, and Acrp30 is an abundant serum protein. Acrp30 may be a factor that participates in the delicately balanced system of energy homeostasis involving food intake and carbohydrate and lipid catabolism. Our experiments also further corroborate the existence of an insulin-regulated secretory pathway in adipocytes.

Phenotypic differences among patients with Bardet-Biedl syndrome linked to three different chromosome loci

Bardet-Biedl syndrome (BBS) is an autosomal-recessive disorder of mental retardation, obesity, retinal dystrophy, polydactyly, and hypogenitalism. Renal and cardiac abnormalities are also frequent in this disorder. Previous clinical suggestions of heterogeneity of BBS were confirmed recently by the identification of four different chromosome loci linked to the disease. In this study we compared clinical manifestations of the syndrome in patients from 3 unrelated, extended Arab-Bedouin kindreds which were used for the linkage mapping of the BBS loci to chromosomes 3, 15, and 16. The observed differences included the limb distribution of the postaxial polydactyly and the extent and age-association of obesity. It appears that the chromosome 3 locus is associated with polydactyly of all four limbs, while polydactyly of the chromosome 15 type is mostly confined to the hands. On the other hand, the chromosome 15 type is associated with early-onset morbid obesity, while the chromosome 16 type appears to present the "leanest" form of BBS. Future cloning of the various BB genes will contribute to the understanding of the molecular basis of limb development and to the identification of human obesity-related genes.

Biological correlates of binge eating

Eating disorders are associated with numerous biological perturbations; however, sorting out cause from effect is difficult. Neuroendocrine and metabolic abnormalities are seen in both anorexia nervosa and bulimia nervosa, but they have not been described in binge eating disorder, in which neither starvation nor compensatory behaviors are present. Although these findings may reflect biologic differences among subgroups of binge eaters, an alternative explanation is that many of the biological correlates of binge eating are the result of metabolic derangement secondary to starvation and/or purging. The identification of binge eating disorder provides an opportunity to study the causes and concomitants of binge eating in the absence of compensatory behaviors.

Weight regain following sustained weight reduction is predicted by relative insulin sensitivity

Ten moderately obese women (body mass index 34.9 +/- 1.1 kg/m2, mean +/- SEM), had previously been through a 3-month weight loss program followed by 3 months of weight maintenance at the reduced weight. A euglycemic clamp for determination of insulin sensitivity was performed on each subject prior to weight loss, and another at the end of the weight maintenance phase. The mean weight loss for the group was 11.4 +/- 2.2 kg. The women were then seen for follow-up weights 12 months and 18 months after the conclusion of the weight maintenance period. All of the women except one had regained their weight by the time of the 12-month visit. It was found that the amount of weight regained both at 12 months and 18 months was correlated with the change in insulin sensitivity which occurred from the baseline study to after weight loss/maintenance. The data indicate that increased insulin sensitivity following sustained weight loss in obese women predicts weight regain.

Acute cold-induced suppression of ob (obese) gene expression in white adipose tissue of mice: mediation by the sympathetic system

The effect of acute exposure to cold on the expression of the ob (obese) gene, which encodes a protein that plays a critical role in the regulation of energy balance and body weight, has been examined in epididymal white adipose tissue of mice. Overnight (18 h) exposure of mice to a temperature of 4 degrees C led to the disappearance of ob mRNA in epididymal white fat, and subsequent studies showed that a cold-induced loss of ob mRNA could occur in as little as 2-4 h of exposure to 4 degrees C. When mice exposed to cold for 18 h were returned to the warm (24 degrees C), there was a rapid stimulation of the expression of the ob gene, the mRNA returning within 2.5 h. Administration of noradrenaline led to a reduction in the level of ob mRNA in mice maintained in the warm, while isoprenaline resulted in the disappearance of the mRNA; these changes in ob mRNA were paralleled by similar changes in lipoprotein lipase mRNA. In contrast to white fat, the level of lipoprotein lipase mRNA in brown adipose tissue was increased by noradrenaline and isoprenaline. It is concluded that there is a cold-induced suppression of ob gene expression in white adipose tissue of mice and that this is mediated primarily by the sympathetic system. The profound effect of cold on ob gene expression indicates that the ob system relates to energy expenditure, as well as to satiety.

Leptin levels in human and rodent: measurement of plasma leptin and ob RNA in obese and weight-reduced subjects

Leptin, the gene product of the obese gene, may play an important role in regulating body weight by signalling the size of the adipose tissue mass. Plasma leptin was found to be highly correlated with body mass index (BMI) in rodents and in 87 lean and obese humans. In humans, there was variability in plasma leptin at each BMI suggesting that there are differences in its secretion rate from fat. Weight loss due to food restriction was associated with a decrease in plasma leptin in samples from mice and obese humans.

New insights into atherosclerosis from studies with mouse models

Atherosclerosis is a disease of the large arteries that is the cause of heart disease and stroke. It is a highly complex disorder with multiple genetic and environmental influences. The mouse model has proved very useful for studying atherosclerosis because genetic analysis and planned genetic modification are feasible in this organism. In this brief review, some recent findings are summarized and future prospects using mouse models to study atherosclerosis-related traits are discussed.

The role of neuropeptide Y in the antiobesity action of the obese gene product

Recently Zhang et al. cloned a gene that is expressed only in adipose tissue of the mouse. The obese phenotype of the ob/ob mouse is linked to a mutation in the obese gene that results in expression of a truncated inactive protein. Human and rat homologues for this gene are known. Previous experiments predict such a hormone to have a hypothalamic target. Hypothalamic neuropeptide Y stimulates food intake, decreases thermogenesis, and increases plasma insulin and corticosterone levels making it a potential target. Here we express the obese protein in Escherichia coli and find that it suppresses food intake and decreases body weight dramatically when administered to normal and ob/ob mice but not db/db (diabetic) mice, which are thought to lack the appropriate receptor. High-affinity binding was detected in the rat hypothalamus. One mechanism by which this protein regulated food intake and metabolism was inhibition of neuropeptide-Y synthesis and release.

Transient increase in obese gene expression after food intake or insulin administration

Obesity is a disorder of energy balance, indicating a chronic disequilibrium between energy intake and expenditure. Recently, the mouse ob gene, and subsequently its human and rat homologues, have been cloned. The ob gene product, leptin, is expressed exclusively in adipose tissue, and appears to be a signalling factor regulating body-weight homeostasis and energy balance. Because the level of ob gene expression might indicate the size of the adipose depot, we suggest that it is regulated by factors modulating adipose tissue size. Here we show that ob gene exhibits diurnal variation, increasing during the night, after rats start eating. This variation was linked to changes in food intake, as fasting prevented the cyclic variation and decreased ob messenger RNA. Furthermore, refeeding fasted rats restored ob mRNA within 4 hours to levels of fed animals. A single insulin injection in fasted animals increased ob mRNA to levels of fed controls. Experiments to control glucose and insulin independently in animals, and studies in primary adipocytes, showed that insulin regulates ob gene expression directly in rats, regardless of its glucose-lowering effects. Whereas the ob gene product, leptin, has been shown to reduce food intake and increase energy expenditure, our data demonstrate that ob gene expression is increased after food ingestion in rats, perhaps through a direct action of insulin on the adipocyte.

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

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

Threading analysis suggests that the obese gene product may be a helical cytokine

The ob gene encodes a protein that, in mutant form, is associated with obesity and type II diabetes in mice. Sequence analysis has revealed no similarities to other proteins, however, and no clues as to possible functions. The possibility nonetheless remains that ob is functionally or ancestrally related to other proteins, whose sequences are divergent to the point that only a comparison of three-dimensional structures might detect relationship. To explore this possibility, we conduct a 'threading' search of a 3-dimensional structure database, to determine whether the ob protein might adopt a fold similar to any known structure. This search reveals that the ob sequence is compatible, at a significance level of P < 0.05, with structures from the family of helical cytokines that includes interleukin-2 and growth hormone. A structural model of ob based upon these results is physically and biologically plausible and leads to testable predictions, including the prediction that ob may activate the JAK-STAT pathway, via binding to a receptor resembling those of the cytokine family.

Physiological control of metabolic flux: the requirement for multisite modulation

Biochemists have long assumed that the flux through a metabolic pathway can be controlled by the activity of a key regulatory enzyme near the beginning of the pathway. We present the accumulating evidence that every step in this assumption is flawed. Instead, effective physiological control of metabolism is shown to involve simultaneous multisite modulation through action on a number of enzymes.

Recombinant ob protein reduces feeding and body weight in the ob/ob mouse

To determine whether the product of the recently cloned ob gene functions as an adipose-related satiety factor, recombinant murine ob protein was administered intraperitoneally to ob/ob mice. Monomeric ob protein given as single morning injections to groups of three animals at seven doses ranging from 5 to 100 micrograms reduced 24-h chow consumption in a dose-dependent manner from values of 81 +/- 6.8% of control (10-micrograms dose, P = 0.04) to 29 +/- 7.7% of control (100-micrograms dose, P < 0.0001). Daily injections of 80 micrograms of ob protein into six ob/ob mice for 2 wk led to an 11 +/- 1.6% decrease in body weight (P = 0.0009) and suppressed feeding to 26 +/- 4.9% of baseline (P < 0.0001), with significant reduction of serum insulin and glucose levels. The effect of recombinant ob protein on feeding was not augmented by cofactors secreted by adipose tissue, nor did exposure of adipose tissue to ob protein affect intracellular ob mRNA levels. Posttranslational modification of ob protein was not required for activity; however, addition of a hexahistidine tag to the amino terminus of the mature ob protein resulted in prolonged suppression of feeding after injection into ob/ob mice. These results demonstrate a direct effect of the ob protein to suppress feeding in the ob/ob mouse and suggest that this molecule plays a critical role in regulating total body fat content.

Regulation of adipocyte gene expression and differentiation by peroxisome proliferator activated receptor gamma

Peroxisome proliferator activated receptor (PPAR)gamma is an orphan member of the nuclear hormone receptor superfamily and is expressed at high levels specifically in adipose tissue. Recent data suggest that this factor is a central regulator of adipocyte gene expression and differentiation. Fibroblastic cell lines that express PPARgamma ectopically can be induced to differentiate into fat cells by a variety of lipids and lipid-like activators of PPARs, suggesting that this protein may function to link adipogenesis with systemic lipid metabolism.

Regulated expression of the obese gene product (leptin) in white adipose tissue and 3T3-L1 adipocytes

A mutation within the obese gene was recently identified as the genetic basis for obesity in the ob/ob mouse. The obese gene product, leptin, is a 16-kDa protein expressed predominantly in adipose tissue. Consistent with leptin's postulated role as an extracellular signaling protein, human embryonic kidney 293 cells transfected with the obese gene secreted leptin with minimal intracellular accumulation. Upon differentiation of 3T3-L1 preadipocytes into adipocytes, the leptin mRNA was expressed concomitant with mRNAs encoding adipocyte marker proteins. A factor(s) present in calf serum markedly activated expression of leptin by fully differentiated 3T3-L1 adipocytes. A 16-hr fast decreased (by approximately 85%) the leptin mRNA level of adipose tissue of lean (ob/+ or +/+) mice but had no effect on the approximately 4-fold higher level in obese (ob/ob) littermates. Since the mutation at the ob locus fails to produce the functional protein, yet its cognate mRNA is overproduced, it appears that leptin is necessary for its own downregulation. Leptin mRNA was also suppressed in adipose tissue of rats during a 16-hr fast and was rapidly induced during a 4-hr refeeding period. Insulin deficiency provoked by streptozotocin also markedly down-regulated leptin mRNA and this suppression was rapidly reversed by insulin. These results suggest that insulin may regulate the expression of leptin.

Diet- and diabetes-induced changes of ob gene expression in rat adipose tissue

ob gene regulation is as yet unknown. We first examined whether the ob gene is under physiological control by the nutritional state. Fasting produced a sharp (95%) decrease of ob mRNA in epididymal and inguinal fat pads from 24 h onward. Refeeding rapidly (3-6 h) re-induced ob gene expression and corrected it within 24 h. Similar changes in fatty acid synthase (FAS) and GLUT4 mRNAs were observed, whereas phosphoenolpyruvate carboxykinase (PEPCK) mRNA showed an opposite evolution. We next examined the potential role of insulin. In adipose tissue of streptozotocin-diabetic rats, ob mRNA levels were decreased by 80%. Insulin treatment (4 days) only marginally increased ob mRNA, but restored euglycemia and overcorrected FAS, GLUT4 and PEPCK expression. In conclusion, we provide evidence for a physiological regulation of ob gene by variations in the nutritional state. We also show that ob expression is impaired in streptozotocin-diabetic rats and only slightly restored by insulin treatment, which suggests that ob gene is not or only minimally regulated by the hormone.

Increased neuropeptide Y secretion in the hypothalamic paraventricular nucleus of obese (fa/fa) Zucker rats

NPY is synthesized in the hypothalamic arcuate nucleus (ARC), and NPY injected into the paraventricular nucleus (PVN), the main site of NPY release, induces hyperphagia and reduces energy expenditure. Hypothalamic NPY mRNA and NPY levels are increased in fatty Zucker rats, consistent with increased NPY release. This could explain the hyperphagia and reduced energy expenditure, which lead to obesity in the fatty Zucker rat. We have therefore compared NPY secretion in the PVN of conscious fatty and lean Zucker rats using push-pull sampling. The NPY secretory profile was consistently higher in fatty Zucker rats than in lean rats throughout the 3-h study period (P < 0.01), and mean NPY secretion over the whole 3 h was increased 2-fold in the fatty rats (P < 0.001). We conclude that fatty Zucker rats have increased NPY release in the PVN. This observation further supports the hypothesis that increased activity of the NPYergic ARC-PVN pathway may contribute to obesity in the fatty Zucker syndrome.

Muscle fibre type and dimension in genetically obese and lean Zucker rats

Skeletal muscle structure and morphology may be altered in obesity. To study this further, muscles from six genetically obese (fa/fa) and six normal male rats were examined at 15 weeks of age. The gluteus medius, vastus lateralis and rectus abdominis muscles were dissected out and stained for histochemical fibre typing. In addition the fibre cross-sectional area was measured on a graphic tablet. The proportion of fast-twitch fibres was larger in the vastus lateralis and rectus abdominis muscles of the obese rats (P < 0.01); no difference was seen for the gluteus medius muscle. For the normal rats the cross-sectional area of the fast-twitch fibres was 2-3 times larger than the area of slow-twitch fibres in the same muscle. The cross-sectional area of the fast-twitch fibres in the obese rats was 40-47% less than in the control animals (P < 0.003), while no difference between the two groups was found for the slow-twitch fibre area. The data thus suggest that in the genetically obese rats the development of fast-twitch fibres was primarily affected. Moreover, in these animals some muscles may be more affected than others.

Expression of ob mRNA and its encoded protein in rodents. Impact of nutrition and obesity

The mutant gene responsible for obesity in the ob/ob mouse was recently identified by positional cloning (Zhang Y., R. Proenca, M. Maffel, M. Barone, L. Leopold, and J.M. Friedman. 1994. Nature (Lond.) 372:425). The encoded protein and to represent and "adipostat" signal reflecting the state of energy stores. We confirm that the adipocyte is the source of ob mRNA and that the predicted 16-kD ob protein is present in rodent serum as detected by Western blot. To evaluate the hypothesis that it might represent an adipostat, we assessed serum levels of ob protein and expression of ob mRNA in adipose cells and tissue of rodents in response to a variety of perturbations which effect body fat mass. Both ob protein and ob mRNA expression are markedly increased in obesity. The levels of ob protein are approximately 5-10-fold elevated in serum of db/db mice, in mice with hypothalamic lesions caused by neonatal administration of monosodium glutamate (MSG), and in mice with toxigene induced brown fat ablation, (UCP-DTA). Very parallel changes are observed in adipocyte ob mRNA expression in these models and in ob/ob mice. As predicted however, no serum ob protein could be detected in the ob/ob mice. By contrast to obesity, starvation of normal rats and mice for 1-3 d markedly suppresses ob mRNA abundance, and this is reversed with refeeding. Similarly, ob protein concentration in normal mice falls to undetectable levels with starvation. In the ob/ob, UCP-DTA and MSG models, overexpression of ob mRNA is reversed by caloric restriction. These data support the hypothesis that expression of ob mRNA and protein are regulated as a function of energy stores, and that ob serves as a circulating feedback signal to sites involved in regulation of energy homeostasis.

Molecular cloning of rat obese cDNA and augmented gene expression in genetically obese Zucker fatty (fa/fa) rats

The obese (ob) gene has recently been isolated through a positional cloning approach, the mutation of which causes a marked hereditary obesity and diabetes mellitus in mice. In the present study, we isolated rat ob cDNA and examined the tissue distribution of the ob gene expression in rats. We also studied the gene expression in genetically obese Zucker fatty (fa/fa) rats. The rat ob gene product, a 167 amino acid protein with a putative signal sequence, was 96 and 83% homologous to the mouse and human ob proteins, respectively. Northern blot analysis using the rat ob cDNA probe identified a single mRNA species of 4.5 kb in size in the adipose tissue, while no significant amount of ob mRNA was present in other tissues in rats. The ob gene was expressed in the adipose tissue with region specificities. The rank order of the ob mRNA level in the adipose tissue was epididymal, retroperitoneal, and pericardial white adipose tissue > mesenteric and subcutaneous white adipose tissue > or = interscapular brown adipose tissue. The ob gene expression occurred in mature adipocytes rather than in stromalvascular cells isolated from the rat adipose tissue. Expression of the ob gene was markedly augmented in all the adipose tissue examined in Zucker fatty (fa/fa) rats at the stage of established obesity. The present study leads to the better understanding of the physiologic and pathophysiologic roles of the ob gene.

The genetics of obesity

Genetic and environmental factors both affect the development of human obesity. The prevalence of obesity has increased over the past 30 years, and changes in the environment must have played a key role in this increase. Studies of monozygotic twins have found differences in body weight that must be due to environmental influences. However, there is also considerable evidence suggesting a genetic basis for obesity. The body mass index (BMI) of adult offspring is correlated with the BMI of parents, and this can be entirely ascribed to the transmitted genes. Thus, the similarity of BMI is about twice as great among monozygotic twins as among dizygotic twins. Moreover, adoption studies have shown a correlation between the BMIs of biological parents, siblings, and adult adoptees, while the BMI of the adult adoptees showed no correlation with the BMI of their adoptive parents. A few major genes may contribute to the development of obesity. Genetic linkage and candidate gene studies have attempted to identify the genes involved in determining BMI in humans, but have so far produced mixed results.

Overexpression of the obese (ob) gene in adipose tissue of human obese subjects

Obesity is accompanied by complications such as hypertension, non-insulin-dependent diabetes mellitus and atherosclerosis, which in turn cause ischaemic heart disease, stroke and premature death. The underlying mechanisms behind imbalance in energy intake and energy expenditure that lead to obesity are still controversial. In most populations, obesity is more common among women than men and is a multifactorial phenotype, which may result from a complex network of genetic and nongenetic factors. The relative importance of genetic factors for obesity is under debate. Genome searches using polymorphic markers in inbred mice with phenotypes that result in extreme obesity and studies of human candidate genes are being performed in an attempt to identify genes that contribute to obesity. There is evidence that body weight is physiologically regulated and it has been postulated that the storage of fat may provide signals to the brain that the body is obese, which in turn may make the subject eat less and burn more fuel. One of the molecules that may be involved in such signalling is the obese (ob) gene product. Mutations in ob result in profound obesity and type II diabetes in mice. The mouse ob gene and its human homologue have been cloned and sequenced. The gene is expressed in adipose tissue and the product has features of a secreted protein. We have investigated human ob expression in subcutaneous and omental adipose tissue obtained from non-obese and massively obese subjects using in situ hybridization histochemistry and report on overexpression in obese people.

Genetic variation on chromosome 1 associated with variation in body fat distribution in men

BACKGROUND

Interindividual variation in fat deposition in swine is determined by loci on porcine chromosome 4, which are contained in a region that is syntenic with part of the long arm of human chromosome 1. We hypothesized that genomic variation of chromosome 1q would be associated with variation in the ratio of waist-to-hip circumference in male North American Hutterites, a genetic isolate characterized by significant relatedness and sharing of environmental factors.

METHODS AND RESULTS

In 316 male Hutterites, we tested for phenotype-genotype association of two DNA polymorphisms on chromosome 1q and the ratio of waist-to-hip circumference. We included control loci on 10 other chromosomes in the multivariate model. We observed that DNA variation on chromosome 1q was significantly associated with variation in the ratio of waist-to-hip circumference in men (P = .0029).

CONCLUSIONS

The association of DNA variation chromosome 1q with the ratio of waist-to-hip circumference in male Hutterites suggests that there are important structural elements in this genomic region that have a functional impact on body fat distribution.

Increased obese mRNA expression in omental fat cells from massively obese humans

Obesity presents a significant challenge to the general health of affluent nations in terms of the number of people affected, the serious associated maladies and the lack of effective treatments. While common wisdom has held that obesity results from 'gluttony and sloth', a number of studies have indicated physiological causes of underlying the pathogenesis of obesity, with the degree of adiposity having a strong genetic component. Recently, the obese gene in the ob/ob mouse was cloned, along with its human homologue. The specific production of the obese protein by adipose tissue suggested that it may function in a feedback loop from fat tissue to the hypothalamus to control energy intake and/or energy expenditure, and that it may play a role in the pathogenesis of human obesity. In this study we report that obese mRNA expression is elevated in ex vivo omental adipocytes isolated from massively obese humans in the absence of an identifiable mutation. Therefore, we speculate that this increased expression may suggest that the massively obese are insensitive to the putative regulatory function(s) of the obese gene product.

Obesity in Britain: gluttony or sloth?

The prevalence of clinical obesity in Britain has doubled in the past decade. The Health of the Nation initiative has set ambitious targets for reversing the trend in recognition of the serious health burden which will accrue, but efforts to develop prevention and treatment strategies are handicapped by uncertainty as to the aetiology of the problem. It is generally assumed that ready access to highly palatable foods induces excess consumption and that obesity is caused by simple gluttony. There is evidence that a high fat diet does override normal satiety mechanisms. However, average recorded energy intake in Britain has declined substantially as obesity rates have escalated. The implication is that levels of physical activity, and hence energy needs, have declined even faster. Evidence suggests that modern inactive lifestyles are at least as important as diet in the aetiology of obesity and possibly represent the dominant factor.

Association of a polymorphism in the beta 3-adrenergic-receptor gene with features of the insulin resistance syndrome in Finns

BACKGROUND

Because visceral obesity predicts insulin resistance, we studied whether alterations in the gene encoding for the beta 3-adrenergic receptor in visceral fat are associated with insulin resistance.

METHODS

We studied the frequency of a cytosine-to-thymidine mutation that results in the replacement of tryptophan by arginine at position 64 (Trp64Arg) of the beta 3-adrenergic receptor by restriction-enzyme digestion with BstOl in 335 subjects from western Finland, 207 of whom were nondiabetic and 128 of whom had non-insulin-dependent diabetes mellitus (NIDDM). We also determined the frequency of the mutation in 156 subjects from southern Finland. Sensitivity to insulin was measured by the hyperinsulinemic-euglycemic clamp technique in 66 randomly selected nondiabetic subjects.

RESULTS

In the subjects from western Finland, the frequency of the mutated allele was similar in the nondiabetic subjects and the subjects with NIDDM (12 vs. 11 percent). The mean age of the subjects at the onset of diabetes was lower among those with the mutation than those without it (56 vs. 61 years, P = 0.04). Among the nondiabetic subjects, those with the mutation had a higher ratio of waist to hip circumference (P = 0.02), a greater increase in the serum insulin response after the oral administration of glucose (P = 0.05), a higher diastolic blood pressure (82 vs. 78 mm Hg, P = 0.01), and a lower rate of glucose disposal during the clamp study (5.3 vs. 6.5 mg [29 vs. 36 mumol] per kilogram of body weight per minute; P = 0.04) than the subjects without the mutated allele. In an analysis of sibling pairs, the siblings with the mutation generally had higher waist:hip ratios (P = 0.05) and higher responses of blood glucose and serum insulin after the oral administration of glucose than their siblings without the mutation (P = 0.02 and P = 0.005, respectively).

CONCLUSIONS

The Trp64Arg allele of the beta 3-adrenergic receptor is associated with abdominal obesity and resistance to insulin and may contribute to the early onset of NIDDM:

The human obese (OB) gene: RNA expression pattern and mapping on the physical, cytogenetic, and genetic maps of chromosome 7

The recently identified mouse obese (ob) gene apparently encodes a secreted protein that may function in the signaling pathway of adipose tissue. Mutations in the mouse ob gene are associated with the early development of gross obesity. A detailed knowledge concerning the RNA expression pattern and precise genomic location of the human homolog, the OB gene, would facilitate examination of the role of this gene in the inheritance of human obesity. Northern blot analysis revealed that OB RNA is present at a high level in adipose tissue but at much lower levels in placenta and heart. OB RNA is undetectable in a wide range of other tissues. Comparative mapping of mouse and human DNA indicated that the ob gene is located within a region of mouse chromosome 6 that is homologous to a portion of human chromosome 7q. We mapped the human OB gene on a yeast artificial chromosome (YAC) contig from chromosome 7q31.3 that contains 43 clones and 19 sequence-tagged sites (STSs). Among the 19 STSs are eight corresponding to microsatellite-type genetic markers, including seven (CA)n repeat-type Genethon markers. Because of their close physical proximity to the human OB gene, these eight genetic markers represent valuable tools for analyzing families with evidence of hereditary obesity and for investigating the possible association between OB mutations and human obesity.

The role of energy expenditure in energy regulation: findings from a decade of research

The role of energy expenditure in energy regulation remains a subject of continuing controversy. New data have emerged from studies conducted over the last decade demonstrating that energy expenditure is a critical factor contributing to successful energy regulation in normal individuals, as well as to the disregulation of energy balance that characterizes obesity. Reduced energy expenditure appears to facilitate weight gain in individuals susceptible to obesity and also appears to reduce the extent of body energy loss during undereating in both lean and obese individuals. The magnitude of the reduction in energy expenditure during, and perhaps after, weight loss is greater than expected on the basis of the reduction in body weight and appears to occur in response to undefined underlying determinants of energy regulation. In addition, exercise intervention studies and cross-sectional investigations of the relationship between energy expenditure for physical activity and body composition demonstrate an apparent equilibration between physical activity and body fat content. This equilibration is suggestive of a direct influence of physical activity on the underlying metabolic determinants of energy balance.

Weight-reducing effects of the plasma protein encoded by the obese gene

The gene product of the ob locus is important in the regulation of body weight. The ob product was shown to be present as a 16-kilodalton protein in mouse and human plasma but was undetectable in plasma from C57BL/6J ob/ob mice. Plasma levels of this protein were increased in diabetic (db) mice, a mutant thought to be resistant to the effects of ob. Daily intraperitoneal injections of either mouse or human recombinant OB protein reduced the body weight of ob/ob mice by 30 percent after 2 weeks of treatment with no apparent toxicity but had no effect on db/db mice. The protein reduced food intake and increased energy expenditure in ob/ob mice. Injections of wild-type mice twice daily with the mouse protein resulted in a sustained 12 percent weight loss, decreased food intake, and a reduction of body fat from 12.2 to 0.7 percent. These data suggest that the OB protein serves an endocrine function to regulate body fat stores.