Positional cloning of the mouse obese gene and its human homologue

Gp130 and the interleukin-6 family of cytokines

Receptors for most interleukins and cytokines that regulate immune and hematopoietic systems belong to the class I cytokine receptor family. These molecules form multichain receptor complexes in order to exhibit high-affinity binding to, and mediate biological functions of, their respective cytokines. In most cases, these functional receptor complexes share common signal transducing receptor components that are also in the class I cytokine receptor family, i.e. gp130, common beta, and common gamma molecules. Interleukin-6 and related cytokines, interleukin-11, leukemia inhibitory factor, oncostatin M, ciliary neurotrophic factor, and cardiotrophin-1 are all pleiotropic and exhibit overlapping biological functions. Functional receptor complexes for this interleukin-6 family of cytokines share gp130 as a component critical for signal transduction. Unlike cytokines sharing common beta and common gamma chains that mainly function in hematopoietic and lymphoid cell systems, the interleukin-6 family of cytokines function extensively outside these systems as well, e.g. from the cardiovascular to the nervous system, owing to ubiquitously expressed gp130. Stimulation of cells with the interleukin-6 family of cytokines triggers homo- or hetero-dimerization of gp130. Although gp130 and its dimer partners possess no intrinsic tyrosine kinase domain, the dimerization of gp130 leads to activation of associated cytoplasmic tyrosine kinases and subsequent modification of transcription factors. This paper reviews recent progress in the study of the interleukin-6 family of cytokines and gp130.

Beta-3 adrenoceptor (beta-3AR) expression in leptin treated OB/OB mice

Leptin-deficient Ob/Ob mice are hypometabolic and have reduced fat cell expression of beta-3 adrenoceptors (ARs). To determine whether leptin repletion restores beta-3 AR number, C57BL/6J Ob/Ob mice were given exogenous leptin (5 mg/kg I.P. daily) for 21 days. Leptin administration reduced body weight from 43.1+/-3.7 to 34.1+/-3.7 g in Ob/Ob animals but had no effect on weight in wildtype animals. Body weight increased by 12% in Ob/Ob mice receiving saline. Beta-3 AR mRNA concentrations were markedly reduced in Ob/Ob animals at baseline. Leptin increased beta-3 AR mRNA to control levels in Ob/Ob mice, but had no effect in wildtype animals. Adipocyte leptin mRNA was increased by 400% in Ob/Ob mice and did not suppress with exogenous leptin administration, suggesting no direct feedback regulation of leptin synthesis. We speculate that restoration of beta-3 AR expression by repleting leptin may be important in correcting hypometabolism in Ob/Ob animals.

Insulin-stimulated translocation of GLUT4 glucose transporters requires SNARE-complex proteins

A major physiological role of insulin is the regulation of glucose uptake into skeletal and cardiac muscle and adipose tissue, mediated by an insulin-stimulated translocation of GLUT4 glucose transporters from an intracellular vesicular pool to the plasma membrane. This process is similar to the regulated docking and fusion of vesicles in neuroendocrine cells, a process that involves SNARE-complex proteins. Recently, several SNARE proteins were found in adipocytes: vesicle-associated membrane protein (VAMP-2), its related homologue cellubrevin, and syntaxin-4. In this report we show that treatment of permeabilized 3T3-L1 adipocytes with botulinum neurotoxin D, which selectively cleaves VAMP-2 and cellubrevin, inhibited the ability of insulin to stimulate translocation of GLUT4 vesicles to the plasma membrane. Furthermore, treatment of the permeabilized adipocytes with glutathione S-transferase fusion proteins encoding soluble forms of VAMP-2 or syntaxin-4 also effectively blocked insulin-regulated GLUT4 translocation. These results provide evidence of a functional role for SNARE-complex proteins in insulin-stimulated glucose uptake and suggest that adipocytes utilize a mechanism of regulating vesicle docking and fusion analogous to that found in neuroendocrine tissues.

Short photoperiod reduces leptin gene expression in white and brown adipose tissue of Djungarian hamsters

Leptin gene expression in white (WAT) and brown adipose tissue (BAT) of the Djungarian hamster (Phodopus sungorus) was analyzed during seasonal acclimatization. Leptin gene expression in WAT was markedly reduced during winter, independent of changes in environmental temperature. Exposure to artificial short photoperiod also decreased leptin gene expression in WATas well as in BAT. Although specific leptin gene expression was lower in BAT, total depot expression was as high as in WAT depots, due to higher RNA content of BAT. Our results indicate that there is significant leptin synthesis in brown fat and that leptin might be involved in photoperiod mediated seasonal adaptations of mammals independent of food deprivation or overfeeding.

Disappearance of body fat in normal rats induced by adenovirus-mediated leptin gene therapy

Sustained hyperleptinemia of 8 ng/ml was induced for 28 days in normal Wistar rats by infusing a recombinant adenovirus containing the rat leptin cDNA (AdCMV-leptin). Hyperleptinemic rats exhibited a 30-50% reduction in food intake and gained only 22 g over the experimental period versus 115-132 g in control animals that received saline infusions or a recombinant virus containing the beta-galactosidase gene (AdCMV-beta Gal). Body fat was absent in hyperleptinemic rats, whereas control rats pair-fed to the hyperleptinemic rats retained approximately 50% body fat. Further, plasma triglycerides and insulin levels were significantly lower in hyperleptinemic versus pair-fed controls, while fatty acid and glucose levels were similar in the two groups, suggestive of enhanced insulin sensitivity in the hyperleptinemic animals. Thus, despite equivalent reductions in food intake and weight gain in hyperleptinemic and pair-fed animals, identifiable fat tissue was completely ablated only in the former group, raising the possibility of a specific lipoatrophic activity for leptin.

Leptin can induce proliferation, differentiation, and functional activation of hemopoietic cells

Many cytokines exert their biological effect through members of the hemopoietin receptor family. Using degenerate oligonucleotides to the common WSXWS motif, we have cloned from human hemopoietic cell cDNA libraries various forms of the receptor that was recently shown to bind the obesity hormone, leptin. mRNAs encoding long and short forms of the human leptin receptor were found to be coexpressed in a range of human and murine hemopoietic organs, and a subset of cells from these tissues bound leptin at the cell surface. Ectopic expression in murine Ba/F3 and M1 cell lines revealed that the long, but not the short, form of the leptin receptor can signal proliferation and differentiation, respectively. In cultures of murine or human marrow cells, human leptin exhibited no capacity to stimulate cell survival or proliferation, but it enhanced cytokine production and phagocytosis of Leishmania parasites by murine peritoneal macrophages. Our data provide evidence that, in addition to its role in fat regulation, leptin may also be able to regulate aspects of hemopoiesis and macrophage function.

Correction of obesity and diabetes in genetically obese mice by leptin gene therapy

The ob/ob mouse is genetically deficient in leptin and exhibits both an obese and a mild non-insulin-dependent diabetic phenotype. To test the hypothesis that correction of the obese phenotype by leptin gene therapy will lead to the spontaneous correction of the diabetic phenotype, the ob/ob mouse was treated with a recombinant adenovirus expressing the mouse leptin cDNA. Treatment resulted in dramatic reductions in both food intake and body weight, as well as the normalization of serum insulin levels and glucose tolerance. The subsequent diminishment in serum leptin levels resulted in the rapid resumption of food intake and a gradual gain of body weight, which correlated with the gradual return of hyperinsulinemia and insulin resistance. These results not only demonstrated that the obese and diabetic phenotypes in the adult ob/ob mice are corrected by leptin gene treatment but also provide confirming evidence that body weight control may be critical in the long-term management of non-insulin-dependent diabetes mellitus in obese patients.

Interaction of nuclear factors with the cAMP response elements of the human β(3)-adrenoceptor gene

Four potential cyclic adenosine 3',5'-monophosphate (cAMP) response elements (CREs), each having at most two mismatches from the classical canonical sequence, have been identified in the 5'UTR of the human β(3)-adrenoceptor gene by Liggett and Schwinn (1991). Recently, three of these CREs were shown to confer responsiveness to cAMP when cloned into a CAT reporter vector (Thomas et al., 1992). In this study, in vitro gel-retardation assays have shown that recombinant human CRE binding protein-1 (CREB-1) or activating transcription factor-1 (ATF-1) can interact specifically with these four putative CREs (termed β(3)CRE2), although with different affinities. Nuclear extracts from human brown or white adipose tissue contain proteins interacting with β(3)CRE3 and β(3)CRE2. These adipose nuclear factors were shown by competition assays and the use of antibodies to differ from CREB-1 or ATF-1. The nuclear factor(s) interacting with β(3)CRE2 was found in human and rat brown and white adipose tissues, but not in the other nonadipose tissues examined, i.e., rat lung, liver, kidney, and heart, suggesting an adipose tissue-specific DNA binding or expression pattern. β(3)CRE2 is found to constitute a hexameric element that is highly homologous to the binding site for members of the nuclear hormone receptor superfamily, and a competition assay using this site has provided evidence that an adipose tissuespecific orphan member of this superfamily may bind to β(3)CRE2. Reporter gene assays have indicated that β(3)CRE2 and β(3)CRE3 slightly repress the basal level of transcription and that β(3)CRE2 confers cAMP responsiveness, whereas β(3)CRE3 does not.

Medicating the obese patient

The acceptance of medication as a legitimate adjunct to diet and behavior modification in the treatment for obesity is an emerging phenomenon spurred by advances in understanding the biologic basis of body weight regulation and by the demonstration of safe and effective chronic maintenance of weight loss using a pharmacobehavioral approach. The decision to medicate for obesity depends on good clinical judgment based on such considerations as body mass index; body composition; body fat dissociation; age; sex; and comorbid conditions, such as diabetes and hypertension. Several nonadrenergic agents and a serotonergic agent have FDA-approved indications for weight loss. Phenylpropanolamine is available over the counter. Clinical trials support the efficacy of fluoxetine and ephedrine or caffeine in producing weight loss, although these agents do not have FDA-approved indications for treatment for obesity. In addition, new agents are being developed or are anticipated for approval. The use of existing agents in combination and their use adjunctive to diet and behavioral approaches to obesity treatment are fertile areas for research. The expectant attention to this subject is demanded by the imperative that the health in one three people in the United States is adversely affected by obesity.

Exercise as treatment for obesity

In recent times, affluent societies have become less physically active, and this has undoubtedly contributed to the increased incidence of obesity. Formal programs of exercise training can reduce body weight and fat, but, in many cases, the changes produced by exercise are small. When combined with energy restriction, exercise results in little further weight loss, but there is a strong trend for a greater loss of body fat. Thus, during diet-induced weight loss, added exercise seems to accelerate fat loss and maintain lean body mass, a condition which may prevent a decline in RMR. It is becoming increasingly clear that weight loss is better maintained when exercise is part of a weight-reducing program. Furthermore, following a period of diet-induced weight loss, participation in regular exercise amounting to an energy expenditure of more than 1500 kcal/week will result in more successful maintenance of the lesser weight. An emphasis should be placed on adopting life-long habits conducive to weight control and overall health rather than temporary measures for weight loss. A program which encompasses regular physical activity, modest energy intake, and reduced calories from fat has the potential to meet such a goal. Regular physical activity has the potential to reverse insulin resistance, improve cardiovascular function and the blood lipid profile, and control high blood pressure. Overweight individuals can obtain these important benefits even if body weight is not completely normalized during a program of regular physical activity. This should help alleviate problems of diabetes, heart disease, and hypertension often associated with being overweight. Further research is needed to identify more specifically the optimal amount, type, and intensity of exercise needed to produce weight loss or maintain ideal body weight. To date, the best recommendation comes from the American College of Sports Medicine. Persons are urged to engage in regular physical activity which promotes a daily energy expenditure of at least 300 kcal/day and to choose from a variety of activities, in particular, those which are enjoyable and that can be continued for life.

Adipocyte differentiation: a transcriptional regulatory cascade

The adipose cell is now known to play a complex role in energy homeostasis, energy storage and signaling to other tissues concerning the state of energy balance. The past few years have seen an explosive increase in our knowledge of the transcriptional basis of adipocyte differentiation. Factors such as peroxisome proliferator-activated receptor gamma, the CCAAT/enhancer binding protein family members, and adipocyte determination- and differentiation-dependent factor 1 play important regulatory roles in this process. Furthermore, these factors provide a focus for beginning to understand how various hormones and metabolites influence the development of adipose tissue in vivo.

The endocrinology of obesity

Numerous endocrine alterations are associated with obesity (Table 1). The majority of the alterations are secondary to obesity and must be considered simply associated and potentially in the pathogenesis of the complications of obesity. The discovery of new endocrine peptides such as leptin that signal body fat content will increase our understanding of the regulation of body fat content. As a result, therapies will most certainly be developed that are directly targeted at the alterations in endocrine function.

Mouse models of human genetic disease: which mouse is more like a man?

There has always been great interest in animal models of human genetic disease, and mice provide the largest number of examples. A mutation in the homologous gene in mice does not always lead to the same phenotype as is found in man, however. Recent studies made it apparent that one mutation can have markedly different phenotypes when placed on different genetic backgrounds. This variation is due to different alleles at modifying loci in various inbred strains. Thus, if one wishes to obtain the optimal mouse model for a human disease, one needs to choose the correct genetic background as well as the correct mutation.

Potassium channel dysfunction in hypothalamic glucose-receptive neurones of obese Zucker rats

1. We have shown, using intracellular and cell-attached recordings, that glucose-receptive (GR) neurones of obese Zucker rats exhibit abnormal electrophysiological responses to changes in extracellular glucose concentration, whereas GR neurones of lean Zucker and control rats respond normally. 2. In inside-out recordings from obese rat GR neurones it was shown that the 150 pS ATP-sensitive K+ (KATP) and the 160 pS calcium-activated K+ (KCa) channels were absent, whereas both were present in GR neurones of lean Zucker and control rats. 3. The potassium channel most frequently observed in inside-out patches from obese GR neurones was characterized by a conductance of 213 pS, was activated by raising internal calcium and inhibited by application of internal ATP. This channel (which we have termed Kfa) was not observed in lean or control rat GR neurones. 4. Tolbutamide (100 microM) was found to induce no effect or to elicit a small depolarization of obese rat GR neurones in the absence of glucose, in contrast to its clear excitatory actions on control or lean Zucker GR neurones. 5. Intracellular, cell-attached and inside-out recordings from obese rat non-GR neurones showed that there was no alteration in their membrane properties or firing characteristics or in the characteristics of the large-conductance calcium-activated K+ channel (KCa) present in these neurones as compared with lean and control rats. 6. It is concluded that the Kfa channel is specific to GR neurones of obese Zucker rats and that the presence of this channel coupled with the absence of KATP and KCa channels results in the abnormal glucose-sensing response of these neurones.

Leptin and OB-R: body weight regulation by a cytokine receptor

There has been intense recent interest in the molecules and pathways governing mammalian body weight regulation. Leptin (OB), an ancestral member of the cytokine family, is an adipocyte-secreted circulating hormone exhibiting weight regulatory properties. Recently, the leptin receptor (OB-R) was identified and shown to exhibit sequence homology and functional similarity to members of the class I cytokine receptor family. The mechanisms governing OB-R triggering and signal transduction have begun to be elucidated, providing new insight into the pathways controlling mammalian body weight homeostasis.

Genetics of obesity in humans and animal models

Human obesity has a significant genetic component which contributes to the risk for this disorder. The application of molecular genetic techniques to identify these genes using a variety of approaches, including information from animal models, will help clarify the role of specific genes in the etiology of human obesity. Identification of these genetic mechanisms is likely to lead to new approaches, both pharmacologic and nonpharmacologic, for the prevention and treatment of this disease.

Lessons from animal models of obesity

Obesity in animals may result from genetic, dietary, or neuroendocrine perturbations. Study of these models has identified the central systems that regulate food intake and energy expenditure and identified the interdependence of feeding behavior, the autonomic nervous system, and adrenal glucocorticoids in the development of obesity. The animal models of obesity have been influential in showing that adipose tissue is an important secretory tissue. The recent identification of five genes that cause obesity will provide new insight into the physiologic systems that regulate energy balance.

Control of food intake. A physiologically complex, motivated behavioral system

This article has examined the control of food intake as a physiologically complex, motivated behavioral system. During the past four decades, considerable progress has been made in understanding putative signals for hunger, satiation, and satiety, although hunger signals have proven to be more difficult to identify. The putative physiologic controls of food intake include positive and negative sensory feedback; gastric and intestinal distension; the effects of nutrients, nutrient reserves, and metabolism in producing signals to the liver or brain; and peptides and hormones released in the gastrointestinal tract or the brain. However, food intake is not influenced solely by physiologic signals for hunger, satiation, and satiety. To comprehend feeding behavior more thoroughly, current physiologic models must be extended to include modulating factors such as feeding-associated responses adapted through learning processes and the influence of circadian rhythms, which can be dominating over hunger, satiation, and satiety signals.

It's the genes! EST access to human genome content

ESTs or 'expressed sequence tags' are DNA sequences read from both ends of expressed gene fragments. The Merck-WashU EST Project and several other public EST projects are being performed to rapidly discover the complement of human genes, and make them easily accessible. These ESTs are widely used to discover novel members of gene families, to map genes to chromosomes as 'sequence-tagged sites' (STSs), and to identify mutations leading to heritable diseases. Informatic strategies for querying the EST databases are discussed, as well as the strengths and weaknesses of the EST data. There is a compelling need to build on the informatic synthesis of human gene data, and to devise facile methods for determining gene functions.

Modulation of insulin activities by leptin

Leptin mediates its effects on food intake through the hypothalamic form of its receptor OB-R. Variants of OB-R are found in other tissues, but their function is unknown. Here, an OB-R variant was found in human hepatic cells. Exposure of these cells to leptin, at concentrations comparable with those present in obese individuals, caused attenuation of several insulin-induced activities, including tyrosine phosphorylation of the insulin receptor substrate-1 (IRS-1), association of the adapter molecule growth factor receptor-bound protein 2 with IRS-1, and down-regulation of gluconeogenesis. In contrast, leptin increased the activity of IRS-1-associated phosphatidylinositol 3-kinase. These in vitro studies raise the possibility that leptin modulates insulin activities in obese individuals.

European Gene Mapping Project (EUROGEM): breakpoint panels for human chromosomes based on the CEPH reference families. Centre d'Etude du Polymorphisme Humain

Meiotic breakpoint panels for human chromosomes 2, 3, 4, 5, 6, 7, 8, 9, 10, 13, 14, 15, 17, 18, 20 and X were constructed from genotypes from the CEPH reference families. Each recombinant chromosome included has a breakpoint well-supported with reference to defined quantitative criteria. The panels were constructed at both a low-resolution, useful for a first-pass localization, and high-resolution, for a more precise placement. The availability of such panels will reduce the number of genotyping experiments necessary to order new polymorphisms with respect to existing genetic markers. This paper shows only a representative sample of the breakpoints detected. The complete data are available on the World Wide Web (URL http:/(/)www.icnet.uk/axp/hgr/eurogem++ +/HTML/data.html) or by anonymous ftp (ftp.gene.ucl.ac.uk in/pub/eurogem/maps/breakpoints).

Positive selection system to screen for inhibitors of human immunodeficiency virus-1 transcription

We describe a transcription-based assay system to screen for antiviral drugs in vivo. The system consists of two transcription units, a cytomegalovirus promoter driving a reporter gene physically linked to an HIV-1 promoter oriented in the opposite direction. Based on the arrangement of the transcription units, enhanced HIV-1 promoter activity in the presence of the viral transactivating Tat protein downregulates reporter gene expression initiated from the CMV promoter. Inhibitors of HIV-1 transcription relieve the suppression and are identified by an increase in reporter gene expression. This positive selection system allows discrimination between drugs that nonspecifically block cellular functions by cytotoxicity and molecules that specifically repress HIV-1 promoter activity.

Obesity genes and the regulation of body fat content

Physiological investigation has demonstrated that the central nervous system monitors body composition and adjusts energy intake and expenditure to stabilize total adipose tissue mass. Genetic variations in the signalling molecules involved in this regulatory system account for the heritable component of body fat content. The application of molecular techniques to rodent models of Mendelian obesity has resulted in the characterization of five loci at which mutations produce an abnormal accumulation of body fat. The genes at these loci include agouti, which encodes a molecule that antagonizes the binding of alpha melanocyte-stimulating hormone to its receptor; fat, which encodes carboxypeptidase E; tubby, which encodes a putative phosphodiesterase; obese, which encodes a circulating satiety protein; and diabetes, which encodes the receptor for the obese gene product. A more detailed understanding of the functional interrelationships of these genes should lead to important new insights into the causes and potential therapies for human obesity.

Regulation of alternative pathway activation and C3a production by adipose cells

Adipose tissue is a major source of adpisin/factor D of the alternative pathway of complement. Adipose tissue also expresses the two other complement components which are involved in the first activation step of the alternative pathway, factor B and C3, and this step is activated in adipose tissue, producing C3a/Acylation Stimulating Protein (C3a/ASP), a stimulator of triglyceride synthesis. Complement activation is a highly regulated process, however, nothing is known about regulation of complement activation in adipose tissue. To gain insight into the nature of adipose complement activation and its regulation, we have now examined the expression of several complement activation regulatory genes, and analyzed the production of C3a/ASP in lean vs. obese, adpisin-deficient mice. We found that undifferentiated preadipocytes expressed the mRNAs encoding the negative regulatory proteins Crry and factor H, but expression of both genes was decreased upon differentiation. The positive regulator properdin, as well as Crry and factor H, were found in adipose tissue. None of these genes was regulated in murine genetic obesity. To investigate the relative levels of complement activation in lean vs. adpisin-deficient obese mice, we developed a radioimmunoassay for measurement of murine C3a/ASP in plasma. We report that there was no significant difference in the level of C3a in lean vs. obese plasma; however, we found a positive correlation between C3a and plasma triglyceride levels in normal lean mice.

Leptin is related to body fat content in male distance runners

Leptin, the product of the ob gene, has been reported to be related to body fat in humans (Considine et al. N. Engl. J. Med. 334: 292, 1996). However, little is known about the physiology of this putative satiety signal in humans. The purpose of the present study was to determine whether leptin is related to body fat content in relatively lean endurance-trained adults. In addition, the effect of acute exercise on circulating leptin concentration was studied. Thirteen male runners, whose mean age, height, weight, %fat, and maximal oxygen consumption (VO2max) were 32.2 +/- 2.5 yr, 176.2 +/- 1.6 cm, 71.9 +/- 6.9 kg, 9.7 +/- 0.9%, and 62.9 +/- 2.2 ml.kg-1.min-1, respectively, were studied. Blood samples were obtained after an overnight fast and again immediately after the completion of a 20-mile run at 70% VO2max under controlled environmental conditions. Serum leptin was closely related to fat mass (r = 0.92) in the runners. Acute exercise had no detectable effect on serum leptin levels (PRE = 2.19 +/- 0.32 ng/ml, POST = 2.14 +/- 0.36 ng/ml). These data indicate that, even at a biological extreme of body fat, circulating leptin concentration is closely related to fat content. Furthermore, the data suggest that, in trained individuals with low leptin concentrations, acute exhaustive exercise has no immediate effect on circulating leptin concentration.

Serum leptin concentration, obesity, and insulin resistance in Western Samoans: cross sectional study

OBJECTIVE

To measure serum leptin concentrations in the Polynesian population of Western Samoa and to examine epidemiological associations of leptin with anthropometric, demographic, behavioural, and metabolic factors in this population with a high prevalence of obesity and non-insulin dependent diabetes mellitus.

DESIGN

Cross sectional study, leptin concentration being measured in a subgroup of a population based sample.

SUBJECTS

240 Polynesian men and women aged 28-74 years were selected to cover the full range of age, body mass index, and glucose tolerance.

MAIN OUTCOME MEASUREMENTS

Serum leptin, insulin, and glucose concentrations; anthropometric measures; physical activity; and area of residence.

RESULTS

Leptin concentrations were correlated with body mass index (r = 0.80 in men, 0.79 in women) and waist circumference (r = 0.82 in men, 0.78 in women) but less so with waist to hip ratio. At any body mass index, leptin concentration was higher in women than men (geometric mean adjusted for body mass index 15.3 v 3.6 pg/l, P < 0.001). Leptin concentration also correlated with fasting insulin concentration (r = 0.63 in men, 0.64 in women) and insulin concentration 2 hours after a glucose load (r = 0.58 in men, 0.52 in women). These associations remained significant after controlling for body mass index; effects of physical activity and of rural or urban living on leptin concentration were eliminated after adjusting for obesity, except values remained high in urban men. 78% of variance in leptin was explained by a model including fasting insulin concentration, sex, body mass index, and a body mass index by sex interaction term. Similar results were obtained if waist circumference replaced body mass index.

CONCLUSIONS

The strong relation of leptin with obesity is consistent with leptin production being proportional of mass to adipose tissue. The relation with insulin independent of body mass index suggests a possible role for leptin in insulin resistance or hyperinsulinaemia.

Regulation of obese (ob) mRNA and plasma leptin levels in rhesus monkeys. Effects of insulin, body weight, and non-insulin-dependent diabetes mellitus

We have cloned the rhesus monkey obese cDNA and have analyzed its expression in monkeys with a wide range of body weights (lean to very obese) and with or without non-insulin-dependent diabetes mellitus to examine the relationship of ob gene expression to obesity and non-insulin-dependent diabetes mellitus. The sequence of monkey ob protein, excluding the signal peptide, showed 91% identity with the human protein. We observed a significant correlation between the level of ob mRNA and body weight. We also found a significant relationship between ob mRNA and fasting plasma insulin concentration; however, insulin stimulation during a 100-140-min euglycemic/hyperinsulinemic clamp did not result in any changes in ob mRNA levels. Circulating levels of the ob gene product leptin were also significantly correlated with body weight. These results show that ob gene expression is related to body weight and is not acutely regulated by insulin.

Acute insulin administration does not affect plasma leptin levels in lean or obese subjects

Whether leptin levels are related to insulin sensitivity or subject to acute regulation by insulin is not known. In 12 obese [body mass index (BMI) = 34.0 +/- 1.5 kg m-2] and 12 lean (BMI = 22.2 +/- 0.6 kg m-2) non-diabetic subjects, plasma leptin concentrations were measured in the fasting state and during 2 hours of euglycaemic hyperinsulinaemia (approximately 600 pmol L-1). Fasting plasma leptin was significantly higher in obese (26.6 +/- 3.2) than in lean subjects (6.4 +/- 1.2 ng mL-1, P = 0.0001), and in women (21.1 +/- 3.3) than in men (7.3 +/- 2.3 ng mL-1, P = 0.01). In univariate analysis, fasting plasma leptin was strongly related to all anthropometric measures (body weight, fat mass, percent fat mass, waist and hip circumferences). In multiple regression, per cent adiposity, hip circumference and duration of obesity explained 90% of the variability in fasting leptin concentrations. Fasting and stimulated (OGTT) insulin levels, insulin sensitivity (22.6 +/- 1.9 vs 36.7 +/- 2.0 mumol min-1 kg-1 in lean and obese subjects, respectively, P < 0.0001), glucose area, and serum triglycerides were positively related to fasting plasma leptin concentrations; none of these associations, however, was statistically significant after adjusting for BMI. During the clamp, plasma leptin concentrations remained constant in both lean and obese subjects. We conclude that neither insulin levels nor sensitivity relate to leptin levels independently of fat mass, and that leptin is not subject to acute (2 hours) regulation by insulin in lean or obese humans.

Lipid mediators of insulin resistance: ceramide signalling down-regulates GLUT4 gene transcription in 3T3-L1 adipocytes

We have previously demonstrated that chronic exposure of 3T3-L1 adipocytes to tumour necrosis factor-alpha (TNF) resulted in a marked decrease (approximately 90%) in cellular GLUT4 (insulin-responsive glucose transporter) mRNA content as a result of a decreased transcription rate of the GLUT4 gene (approximately 75%) and a reduced half-life of its mRNA (9 to 4.5 h). Investigation of the signalling mechanism responsible for this regulation demonstrated that in the 3T3-L1 adipocytes, sphingomyelin levels decreased to 50% of control levels within 40 min of exposure to TNF, consistent with activation of a sphingomyelinase. In the same manner as with TNF, treatment of the adipocytes with 1-3 microM C6-ceramide, a membrane-permeable analogue of ceramide, decreased GLUT4 mRNA content by approximately 60%. Subsequent investigations revealed that transcription of the GLUT4 gene was reduced by approximately 65% in response to C6-ceramide, demonstrating that the decrease in mRNA content is mediated by a reduction in the transcription of the genc. No effect on GLUT4 mRNA stability was observed after exposure of the adipocytes to C6-ceramide. These observations are interesting in light of our previous data demonstrating that TNF affects both GLUT4 transcription and mRNA stability in the 3T3-L1 adipocytes. In conclusion, the effect of ceramide on GLUT4 gene expression is at the level of transcription, suggesting that another pathway controls mRNA stability. These data establish that ceramide-initiated signal transduction pathways exist within the adipocyte, and provide a potential mechanism for control of GLUT4 gene expression.

Insulin action on glucose transport and plasma membrane GLUT4 content in skeletal muscle from patients with NIDDM

We investigated the response of the glucose transport system to insulin, in the presence of ambient glucose concentrations, in isolated skeletal muscle from seven patients with non-insulin-dependent diabetes mellitus (NIDDM) (age, 55 +/- 3 years, BMI 27.4 +/- 1.8 kg/m2) and seven healthy control subjects (age, 54 +/- 3 years, BMI 26.5 +/- 1.1 kg/m2). Insulin-mediated whole body glucose utilization was similar between the groups when studied in the presence of ambient glucose concentrations (approximately 10 mmol/l for the NIDDM patients and 5 mmol/l for the control subjects). Samples were obtained from the vastus lateralis muscle, by means of an open muscle biopsy procedure, before and after a 40-min insulin infusion. An increase in serum insulin levels from 54 +/- 12 to 588 +/- 42 pmol/l, induced a 1.6 +/- 0.2-fold increase in glucose transporter protein (GLUT4) in skeletal muscle plasma membranes obtained from the control subjects (p < 0.05), whereas no significant increase was noted in plasma membrane fractions prepared from NIDDM muscles, despite a similar increase in serum insulin levels. At concentrations of 5 mmol/l 3-O-methylglucose in vitro, insulin (600 pmol/l) induced a 2.2-fold (p < 0.05) increase in glucose transport in NIDDM muscles and a 3.4-fold (p < 0.001) increase in the control muscles. Insulin-stimulated 3-O-methylglucose transport was positively correlated with whole body insulin-mediated glucose uptake in all participants (r = 0.78, p < 0.001) and negatively correlated with fasting plasma glucose levels in the NIDDM subjects (r = 0.93, p < 0.001). Muscle fibre type distribution and capillarization were similar between the groups. Our results suggest that insulin-stimulated glucose transport in skeletal muscle from patients with NIDDM is down-regulated in the presence of hyperglycaemia. The increased flux of glucose as a consequence of hyperglycaemia may result in resistance to any further insulin-induced gain of GLUT4 at the level of the plasma membrane.

Evidence of free and bound leptin in human circulation. Studies in lean and obese subjects and during short-term fasting

Little is known about leptin's interaction with other circulating proteins which could be important for its biological effects. Sephadex G-100 gel filtration elution profiles of 125I-leptin-serum complex demonstrated 125I-leptin eluting in significant proportion associated with macromolecules. The 125I-leptin binding to circulating macromolecules was specific, reversible, and displaceable with unlabeled leptin (ED50: 0.73 +/- 0.09 nM, mean +/- SEM, n = 3). Several putative leptin binding proteins were detected by leptin-affinity chromatography of which either 80- or 100-kD proteins could be the soluble leptin receptor as approximately 10% of the bound 125I-leptin was immunoprecipitable with leptin receptor antibodies. Significantly higher (P < 0.001) proportions of total leptin circulate in the bound form in lean (46.5 +/- 6.6%) compared with obese (21.4 +/- 3.4%) subjects. In lean subjects with 21% or less body fat, 60-98% of the total leptin was in the bound form. Short-term fasting significantly decreased basal leptin levels in three lean (P < 0.0005) and three obese (P < 0.005) subjects while refeeding restored it to basal levels. The effects of fasting on free leptin levels were more pronounced in lean subjects (basal vs. 24-h fasting: 19.6 +/- 1.9 vs. 1.3 +/- 0.4 ng/ml) compared with those in obese subjects (28.3 +/- 9.8 vs. 14.7 +/- 5.3). No significant (P > 0.05) decrease was observed in bound leptin in either group. These studies suggest that in obese individuals the majority of leptin circulates in free form, presumably bioactive protein, and thus obese subjects are resistant to free leptin. In lean subjects with relatively low adipose tissue, the majority of circulating leptin is in the bound form and thus may not be available to brain receptors for its inhibitory effects on food intake both under normal and food deprivation states.

The cell density factor CMF regulates the chemoattractant receptor cAR1 in Dictyostelium

Starving Dictyostelium cells aggregate by chemotaxis to cAMP when a secreted protein called conditioned medium factor (CMF) reaches a threshold concentration. Cells expressing CMF antisense mRNA fail to aggregate and do not transduce signals from the cAMP receptor. Signal transduction and aggregation are restored by adding recombinant CMF. We show here that two other cAMP-induced events, the formation of a slow dissociating form of the cAMP receptor and the loss of ligand binding, which is the first step of ligand-induced receptor sequestration, also require CMF. Vegetative cells have very few CMF and cAMP receptors, while starved cells possess approximately 40,000 receptors for CMF and cAMP. Transformants overexpressing the cAMP receptor gene cAR1 show a 10-fold increase of [3H]cAMP binding and a similar increase of [125I]CMF binding; disruption of the cAR1 gene abolishes both cAMP and CMF binding. In wild-type cells, downregulation of cAR1 with high levels of cAMP also downregulates CMF binding, and CMF similarly downregulates cAMP and CMF binding. This suggests that the cAMP binding and CMF binding are closely linked. Binding of approximately 200 molecules of CMF to starved cells affects the affinity of the majority of the cAR1 cAMP receptors within 2 min, indicating that an amplifying mechanism allows one activated CMF receptor to regulate many cARs. In cells lacking the G-protein beta subunit, cAMP induces a loss of cAMP binding, but not CMF binding, while CMF induces a reduction of CMF binding without affecting cAMP binding, suggesting that the linkage of the cell density-sensing CMF receptor and the chemoattractant cAMP receptor is through a G-protein.

Demonstration that acid-ethanol extracts of rat adipose tissue contain an inhibitor of food intake in the mouse

The recent identification of the mouse obese (ob) gene, whose product is a approximately 16 kd protein secreted from adipose tissue, and the demonstration that the administration of recombinant OB protein inhibits food intake have led us to report a 1979 pilot study demonstrating an extractable activity from rat adipose tissue that inhibited food intake in the normal mouse. Two hundred grams of rat adipose tissue and 40 gm of rat muscle were extracted with acid-ethanol. The aqueous phase was lyophilized, and a filtered solution of the crude powder was injected subcutaneously daily into normal male mice. Administration of the extract from adipose tissue, but not from muscle tissue, resulted in a significant reduction in food intake and slowing of the rate of increase in body weight over the 10-day experimental period. The quantity of extracted fat represented by the daily dose was equivalent to 1.65 gm of fat administered to a 24-gram mouse. In retrospect this acid-ethanol extractable activity was probably the native OB protein in normal rat adipose tissue.

Adipocyte differentiation is regulated by a prostaglandin liganded to the nuclear peroxisome proliferator-activated receptor

Adipocyte differentiation and the homeostasis of fat cell number are regulated by a nuclear receptor protein, the peroxisome proliferator-activated receptor (PPAR gamma). The endogenous ligand is thought to be 15-deoxy-delta 12,14-prostaglandin J2-Thiazolidinedione, an antidiabetic drug, closely mimics the action of the prostaglandin, suggesting a regulatory role by the prostaglandin and PPAR gamma both in adipocyte differentiation and in glucose homeostasis.

A role for leptin and its cognate receptor in hematopoiesis

BACKGROUND

Hematopoiesis entails the production of multiple blood cell lineages throughout the lifespan of the organism. This is accomplished by the regulated expansion and differentiation of hematopoietic precursors that originate from self-renewing hematopoietic stem cells. Studies of lineage commitment and proliferation have shown that the cytokine family of growth factors plays an important role in hematopoietic differentiation. However, in hematopoiesis, as in most self-renewing biological systems, the molecules that regulate the stem cells directly remain largely unknown. In this study, we have undertaken a search for novel cytokines that may influence the fate of hematopoietic stem cells.

RESULTS

We have cloned three splice variants of a novel cytokine receptor from human hematopoietic stem cells expressing the CD34 antigen, one of which is identical to the leptin receptor. Expression analysis revealed that the leptin receptor is expressed in both human and murine hematopoietic stem cell populations, and that leptin is expressed by hematopoietic stroma. We show that leptin provides a proliferative signal in hematopoietic cells. Importantly, we demonstrate that leptin provides a proliferative signal in BAF-3 cells and increases the proliferation of hematopoietic stem cell populations. The proliferative effects of leptin seem to be at the level of a multilineage progenitor, as shown by increased myelopoiesis, erythropoiesis and lymphopoiesis. Analysis of db/db mice, in which the leptin receptor is truncated, revealed that the steady-state levels of peripheral blood B cells and CD4-expressing T cells were dramatically reduced, demonstrating that the leptin pathway plays an essential role in lymphopoiesis. Colony assays performed using marrow from db/db and wild-type mice indicated that db/db marrow has a deficit in lymphopoietic progenitors; furthermore, db/db mice are unable to fully recover the lymphopoietic population following irradiation insult, and although the levels of peripheral blood erythrocytes are normal in db/db mice, spleen erythrocyte production is severely compromized.

CONCLUSIONS

We have discovered that leptin and its cognate receptor constitute a novel hematopoietic pathway that is required for normal lymphopoiesis. This pathway seems to act at the level of the hematopoietic stem/progenitor cell, and may well also impact upon erythropoiesis, particularly in anemic states that may require output from the spleen. These findings offer a new perspective on the role of the fat cell in hematopoiesis.