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

Trajectory of leptin and leptin receptor in vertebrates: Structure, function and their regulation

The present review provides a comparative insight into structure, function and control of leptin system in fishes, herptiles, birds and mammals. In general, leptin acts as an anorexigenic hormone since its administration results in decrease of food intake in vertebrates. Nonetheless, functional paradox arises in fishes from contradictory observations on level of leptin during fasting and re-feeding. In addition, leptin is shown to modulate metabolic functions in fishes, reptiles, birds and mammals. Leptin also regulates reproductive and immune functions though more studies are warranted in non-mammalian vertebrates. The expression of leptin and its receptor is influenced by numerous factors including sex steroids, stress and stress-induced catecholamines and glucocorticoids though their effect in non-mammalian vertebrates is hard to be generalized due to limited studies.

Insulin as a hormone regulator of the synthesis and release of leptin by white adipose tissue

Leptin and its receptor are widely distributed in several tissues, mainly in white adipose tissue. The serum leptin is highly correlated with body mass index in rodents and humans, being documented that leptin levels reduces in the fasting state and increase during refeeding, similarly to insulin release by pancreatic islets. Insulin appears to increase leptin mRNA and protein expression and its release by adipocytes. Some studies have suggested that insulin acts through the activation of the transcription factors: sterol regulatory element binding protein 1 (SREBP1), CCAAT enhancer binding protein-α (C/EBP-α) and specificity protein 1 (Sp1). Insulin stimulates the release of preformed and newly synthesized leptin by adipocytes through its signaling cascade. Its effects are blocked by inhibitors of the insulin signaling pathway, as well as by inhibitors of protein synthesis and agents that increase the intracellular cAMP. The literature data suggest that chronic hyperinsulinemia increases serum leptin levels in humans and rodents. In this review, we summarized the most updated knowledge on the effects of insulin on serum leptin levels, presenting the cell mechanisms that control leptin synthesis and release by the white adipose tissue.

The Role of Leptin in Maintaining Plasma Glucose During Starvation

For 20 years it has been known that concentrations of leptin, a hormone produced by the white adipose tissue (WAT) largely in proportion to body fat, drops precipitously with starvation, particularly in lean humans and animals. The role of leptin to suppress the thyroid and reproductive axes during a prolonged fast has been well defined; however, the impact of leptin on metabolic regulation has been incompletely understood. However emerging evidence suggests that, in starvation, hypoleptinemia increases activity of the hypothalamic-pituitary-adrenal axis, promoting WAT lipolysis, increasing hepatic acetyl-CoA concentrations, and maintaining euglycemia. In addition, leptin may be largely responsible for mediating a shift from a reliance upon glucose metabolism (absorption and glycogenolysis) to fat metabolism (lipolysis increasing gluconeogenesis) which preserves substrates for the brain, heart, and other critical organs. In this way a leptin-mediated glucose-fatty acid cycle appears to maintain glycemia and permit survival in starvation.

Chicken Is a Useful Model to Investigate the Role of Adipokines in Metabolic and Reproductive Diseases

Reproduction is a complex and essential physiological process required by all species to produce a new generation. This process involves strict hormonal regulation, depending on a connection between the hypothalamus-pituitary-gonadal axis and peripheral organs. Metabolic homeostasis influences the reproductive functions, and its alteration leads to disturbances in the reproductive functions of humans as well as animals. For a long time, adipose tissue has been recognised as an endocrine organ but its ability to secrete and release hormones called adipokines is now emerging. Adipokines have been found to play a major role in the regulation of metabolic and reproductive processes at both central and peripheral levels. Leptin was initially the first adipokine that has been described to be the most involved in the metabolism/reproduction interrelation in mammals. In avian species, the role of leptin is still under debate. Recently, three novel adipokines have been discovered: adiponectin (ADIPOQ, ACRP30), visfatin (NAMPT, PBEF), and chemerin (RARRES2, TIG2). However, their mode of action between mammalian and nonmammalian species is different due to the different reproductive and metabolic systems. Herein, we will provide an overview of the structure and function related to metabolic and reproductive mechanisms of the latter three adipokines with emphasis on avian species.

Primate aging in the mammalian scheme: the puzzle of extreme variation in brain aging

At later ages, humans have high risk of developing Alzheimer disease (AD) which may afflict up to 50% by 90 years. While prosimians and monkeys show more substantial changes, the great apes brains examined show mild neurodegenerative changes. Compared with rodents, primates develop and reproduce slowly and are long lived. The New World primates contain some of the shortest as well as some of the longest-lived monkey species, while the prosimians develop the most rapidly and are the shortest lived. Great apes have the largest brains, slowest development, and longest lives among the primates. All primates share some level of slowly progressive, age-related neurodegenerative changes. However, no species besides humans has yet shown regular drastic neuron loss or cognitive decline approaching clinical grade AD. Several primates accumulate extensive deposits of diffuse amyloid-beta protein (Aβ) but only a prosimian-the gray mouse lemur-regularly develops a tauopathy approaching the neurofibrillary tangles of AD. Compared with monkeys, nonhuman great apes display even milder brain-aging changes, a deeply puzzling observation. The genetic basis for these major species differences in brain aging remains obscure but does not involve the Aβ coding sequence which is identical in nonhuman primates and humans. While chimpanzees merit more study, we note the value of smaller, shorter-lived species such as marmosets and small lemurs for aging studies. A continuing concern for all aging studies employing primates is that relative to laboratory rodents, primate husbandry is in a relatively primitive state, and better husbandry to control infections and obesity is needed for brain aging research.

Intra- and interindividual variation in gene expression in human adipose tissue

Adipose tissue is a highly plastic tissue with an important endocrine and metabolic function. To understand its role in human health and disease, it is necessary to understand the extent of variation and the specific differences within and between different depots and subjects. We employed cDNA microarray analysis to investigate this in human subjects ranging from lean to mildly obese. We observe (1) high similarity between different samples of one adipose depot, (2) only small differences between the subcutaneous and visceral adipose tissue depot and (3) larger differences in gene expression between different individuals (per depot). The major variation within adipose depots can be attributed to differences in the non-adipocyte component of adipose tissue. Using only non-obese subjects, we identified genes that were consistently differentially expressed between subcutaneous and omental adipose tissue, despite the variation in gene expression between these subjects. Using quantitative real time polymerase chain reaction (PCR), fatty acid binding protein 4 (FABP4), vimentin (Vim), four and a half LIMs domains (FHL1), CD36 (all higher in subcutaneous adipose tissue) and Matrix Gla protein (MGP; lower in subcutaneous adipose tissue) were confirmed to be significantly differentially expressed between depots.

Reference body composition in adult rhesus monkeys: glucoregulatory and anthropometric indices

Rhesus monkeys have been used as models to study obesity and disease. The aim of this study was to define body mass indices for underweight and obesity in rhesus monkeys. Longitudinal data collected over 8-14 years from 40 male and 26 female rhesus monkeys were analyzed. Body weight, insulin sensitivity index, and disposition index were regressed against percent body fat (%BF). A minimal %BF beyond which further loss of body weight resulted in loss of lean mass was determined to be 11.5% in older males, 8% in adult females, and 9% in younger adult males. Insulin sensitivity index and disposition index reached minimum values at 23% fat in older males, 18% in adult females, and 21% in younger adult males, indicating obesity. The estimated reference range for %BF was 9%-23% in male and 8%-18% in female monkeys, corresponding to body mass indices of 32-44 kg/m(2) for male and 27-35 kg/m(2) for female monkeys.

HLA-E expression on porcine cells: protection from human NK cytotoxicity depends on peptide loading

Human NK cells lyse porcine cells and may play an important role in the cell-mediated rejection of pig-to-human xenografts. Lysis is probably a consequence of the failure of human MHC-specific killer inhibitory receptors to recognize porcine MHC class I molecules. A majority of activated human NK cells express the HLA-E-specific inhibitory receptor CD94/NKG2A. The aim of this study was therefore to test the hypothesis that stable surface expression of HLA-E on porcine cells protects against xenogeneic NK-mediated cytotoxicity. Porcine lymphoblastoid (13 271) and endothelial (pEC) cell lines were transfected with constructs coding for HLA-E together with the leader sequence of HLA-B7 or -A2. HLA-E was correctly expressed on 13 271 cells while pEC required peptide-pulsing and/or IFN-gamma stimulation to express the HLA-E complex on the cell surface. HLA-E-expressing porcine cells were partially protected from lysis mediated by human polyclonal NK populations and completely protected from killing by NKG2Abright NK clones. In conclusion, the capability of different porcine cell types to express HLA-E on the cell surface can differ considerably depending decisively on the availability of peptides. These findings are important for the applicability of transgenic HLA-E expression as an approach to protect porcine tissues from human NK cytotoxicity.

In vivo, RFX5 binds differently to the human leucocyte antigen-E, -F, and -G gene promoters and participates in HLA class I protein expression in a cell type-dependent manner

We analysed the regulation of human leucocyte antigen (HLA)-E, -F and -G genes, focusing on the SXY module, a promoter region that controls major histocompatibility complex (MHC) class II expression and participates in the expression of classical HLA class I molecules. It comprises the X1, X2 and Y boxes, bound by RFX, X2-BP/ATF/CREB and NFY factors, respectively. The complex recruits the master control factor CIITA. The SXY module is conserved in HLA-E and HLA-F gene promoters, whereas in the HLA-G promoter, the only conserved boxes are S and X1. Chromatin immunoprecipitation assays, performed on HLA-G positive and negative cell lines, demonstrated the in situ binding of RFX5 and CIITA to HLA-E and HLA-F, but not to HLA-G, promoters. In B cells from bare lymphocyte syndrome patients lacking RFX5 or CIITA, we observed lower steady-state levels of HLA-E and HLA-F transcripts but did not find any significant decrease in the cell-surface expression of HLA-E/classical HLA class I. In RFX5-deficient fibroblasts, the cell-surface expression of HLA molecules was decreased. RFX5 and CIITA are thus not involved in HLA-G expression and their importance for the surface expression of HLA-E/classical HLA class I molecules may vary depending on the cell type.

The effects of body fat on pulmonary function and gas exchange in cynomolgus monkeys

Obesity adversely affects lung function in humans often reducing arterial blood oxygenation. To determine if obesity adversely affects lung function in cynomolgus monkeys, which is a species that is often used for pulmonary research, pulmonary mechanics, ventilation, functional residual capacity (FRC), and arterial blood gases were measured using spontaneous respiration and on mechanical ventilation with room air or 100% O(2). Body fat percentage was measured by dual energy X-ray absorption. Blood leptin levels were measured by radioimmune assay. Obese monkeys breathed faster with lower tidal volume, but pulmonary resistance and dynamic lung compliance did not change with body fat. FRC and blood leptin were, respectively, negatively and positively correlated with percent body fat. FRC correlated moderately with ventilatory parameters and strongly with arterial oxygen tension, alveolar-arterial oxygen difference and venous admixture. Therefore, obesity in cynomolgus monkeys had marked, deleterious effects on FRC, ventilation and arterial oxygenation. Obesity may be an important confounding variable in lung function studies in primates.

Genetics of leptin expression in baboons

OBJECTIVE

Leptin gene expression is higher in females than in males, and is regulated by many factors including energy intake and insulin, but little is known about the inheritance of leptin gene expression. We have investigated leptin (LEP) gene express-ion, to determine whether it is heritable, and whether the difference in LEP expression between males and females has a genetic component.

STUDY POPULATION

A total of 319 baboons (Papio hamadryas) (220 females, 99 males) from a captive, pedigreed colony.

MEASUREMENTS AND METHODS

We cloned a baboon LEP cDNA, and quantified LEP mRNA expression in baboon omental adipose tissue using a ribonuclease protection assay. In addition, we assayed circulating leptin levels, adipocyte cell volume, and weight. We used maximum likelihood-based variance decomposition methods to determine the genetic architecture of LEP levels, including testing for genotype-by-sex interaction.

RESULTS

Omental LEP mRNA expression was significantly and positively correlated with weight and adipocyte cell volume in baboons. Both mRNA and plasma levels of leptin were higher in females than in males, and both measures were heritable. The results of our genetic analysis show that there was a genotype-by-sex interaction in the levels of plasma leptin, but not in omental LEP mRNA.

CONCLUSIONS

As in humans, baboon leptin mRNA and protein levels are expressed at a higher level in females than in males. We detected evidence that the plasma levels were affected by genes that are differentially expressed in males and females, while the omental mRNA levels were not. This finding suggests that the genes that differentially regulate plasma leptin levels between males and females may exert their effects on post-transcriptional processes.

Nuclear factor 90 mediates activation of the cellular antiviral expression cascade

Viral infection triggers a cascade of interferon response genes, but the mechanisms that prime such innate antiviral defenses are poorly understood. Among candidate cellular mediators of the antiviral response are the double-stranded RNA (dsRNA)-binding proteins. Here we show that a C-terminal variant of the ubiquitous dsRNA-binding protein, nuclear factor 90 (NF90ctv), can activate the interferon response genes in the absence of viral infection. NF90ctv-expressing cells were infected with the syncytium-inducing HIV-1 strain NL4-3 and were shown to inhibit viral replication. To gain insight into this mechanism of protection, we analyzed the expression profiles of NF90ctv-positive cells as compared with parental cells transduced with the empty vector. Of the 5600 genes represented on the expression arrays, 90 displayed significant (4-fold or more) changes in mRNA levels in NF90-expressing cells. About 50% are known interferon alpha/beta-stimulated genes. The microarray expression data were confirmed by quantitative reverse transcriptase-polymerase chain reaction analysis of six representative interferon-inducible genes. Electrophoretic mobility shift assays showed that the biological response is mediated by the activation of transcription factors in NF90ctv-expressing cells. Functional significance of the activated transcription complex was evaluated by transfection assays with luciferase reporter constructs driven by the interferon-inducible promoter from the 2'-5'-oligoadenylate synthetase (p69) gene. Resistance to HIV-1, caused by the expression of NF90ctv in the cell culture system, appears to be mediated in part by the induction of interferon response genes. This leads to a hypothesis as to the mechanism of action of NF90 in mediating endogenous antiviral responses.

IFN-gamma downregulates interleukin-4 functional activity on monocytes by multiple mechanisms

Interleukin-4 (IL-4) has potent anti-inflammatory properties on monocytes and suppresses lipopolysaccharide (LPS)-induced tumor necrosis factor-alpha (TNF-alpha) and IL-1beta production. Culture with interferon (IFN-gamma) alters human monocyte responses to IL-4 by multiple mechanisms. As previously published, IFN-gamma reduced IL-4-activated signal transducer and activator of transcription-6 (STAT-6). This correlated with an inability of IL-4 to suppress LPS-induced TNF-alpha but not IL-1beta production. A second mechanism, apparent some 48 h after exposure to IFN-gamma, involved a significant suppression of IL-4 receptor (IL-4R) expression at the cell surface, and this correlated with the loss of additional functional responses to IL-4, including IL-4-induced suppression of LPS-induced IL-1beta production. This study identified a further role of IFN-gamma on IL-4 responses, including reduced IL-4R surface expression by human monocytes. Increased release of soluble gammac from IFN-gamma-treated monocytes provides an additional mechanism by which IFN-gamma may control the functional activity of IL-4. This study characterizes further the opposing effects of the type 1 and type 2 cytokine regulatory systems.

Leptin: a possible metabolic signal affecting reproduction

Since its discovery in 1994, leptin, a protein hormone synthesized and secreted by adipose tissue, has been shown to regulate feed intake in several species including sheep and pigs. Although a nimiety of information exists regarding the physiological role of leptin in rodents and humans, the regulation and action of leptin in domestic animals is less certain. Emerging evidence in several species indicates that leptin may also affect the hypothalamo-pituitary-gonadal axis. Leptin receptor mRNA is present in the anterior pituitary and hypothalamus of several species, including sheep. In rats, effects of leptin on GnRH, LH and FSH secretion have been inconsistent, with leptin exhibiting both stimulatory and inhibitory action in vivo and in vitro. Evidence to support direct action of leptin at the level of the gonad indicates that the leptin receptor and its mRNA are present in ovarian tissue of several species, including cattle. These leptin receptors are functional, since leptin inhibits insulin-induced steroidogenesis of both granulosa and thecal cells of cattle in vitro. Leptin receptor mRNA is also found in the testes of rodents. As with the ovary, these receptors are functional, at least in rats, since leptin inhibits hCG-induced testosterone secretion by Leydig cells in vitro. During pregnancy, placental production of leptin may be a major contributor to the increase in maternal leptin in primates but not rodents. However, in both primates and rodents, leptin receptors exist in placental tissues and may regulate metabolism of the fetal-placental unit. As specific leptin immunoassays are developed for domestic animals, in vivo associations may then be made among leptin, body energy stores, dietary energy intake and reproductive function. This may lead to a more definitive role of leptin in domestic animal reproduction.

Galectin-12, an Adipose-expressed Galectin-like Molecule Possessing Apoptosis-inducing Activity

Galectins constitute a family of proteins that bind to beta-galactoside residues and have diverse physiological functions. Here we report on the identification of a galectin-like molecule, galectin-12, in a human adipose tissue cDNA library. The protein contained two potential carbohydrate-recognition domains with the second carbohydrate-recognition domain being less conserved compared with other galectins. In vitro translated galectin-12 bound to a lactosyl-agarose column far less efficiently than galectin-8. Galectin-12 mRNA was predominantly expressed in adipose tissue of human and mouse and in differentiated 3T3-L1 adipocytes. Caloric restriction and treatment of obese animals with troglitazone increased galectin-12 mRNA levels and decreased the average size of the cells in adipose tissue. The induction of galectin-12 expression by the thiazolidinedione, troglitazone, was paralleled by an increase in the number of apoptotic cells in adipose tissue. Immunocytochemical analysis revealed that galectin-12 was localized in the nucleus of adipocytes, and transfection with galectin-12 cDNA induced apoptosis of COS-1 cells. These results suggest that galectin-12, an adipose-expressed galectin-like molecule, may participate in the apoptosis of adipocytes.

Circulating concentrations of the adipocyte protein adiponectin are decreased in parallel with reduced insulin sensitivity during the progression to type 2 diabetes in rhesus monkeys

Adiponectin is an adipose-specific plasma protein whose plasma concentrations are decreased in obese subjects and type 2 diabetic patients. This protein possesses putative antiatherogenic and anti-inflammatory properties. In the current study, we have analyzed the relationship between adiponectin and insulin resistance in rhesus monkeys (Macaca mulatta), which spontaneously develop obesity and which subsequently frequently progress to overt type 2 diabetes. The plasma levels of adiponectin were decreased in obese and diabetic monkeys as in humans. Prospective longitudinal studies revealed that the plasma levels of adiponectin declined at an early phase of obesity and remained decreased after the development of type 2 diabetes. Hyperinsulinemic-euglycemic clamp studies revealed that the obese monkeys with lower plasma adiponectin showed significantly lower insulin-stimulated peripheral glucose uptake (M rate). The plasma levels of adiponectin were significantly correlated to M rate (r = 0.66, P < 0.001). Longitudinally, the plasma adiponectin decreased in parallel to the progression of insulin resistance. No clear association was found between the plasma levels of adiponectin and its mRNA levels in adipose tissue. These results suggest that reduction in circulating adiponectin may be related to the development of insulin resistance.

Cloning of leptin cDNA and assignment to the long arm of chromosome 5 in the marsupial Sminthopsis crassicaudata

We have isolated and sequenced full-length cDNA clones for leptin in the dasyurid marsupial Sminthopsis crassicaudata (fat-tailed dunnart). Southern and in situ hybridisation data indicated a single leptin gene in the S. crassicauda- ta genome, localised to arbitrary chromosome bands 5q24--> q31 on the long arm of chromosome 5, the short-arm terminus of which bears the only nucleolar organising region. The nucleotide sequence of the cDNAs revealed that the primary translation product of S. crassicaudata leptin is composed of 167 amino acid residues, with a potential signal peptide of 21 residues. The mature protein of 146 amino acids is 82% similar to both the mouse and human proteins and is predicted to have a molecular weight of 16.26 kDa. Northern blot analysis revealed that the corresponding mRNA is approximately 3.9 kb in size and is expressed only in white adipose tissue of this marsupial species. Evolutionary analyses indicate that S. crassicaudata leptin cDNA has evolved at a significantly faster rate than cDNAs from eutherian mammals.

Dual effect of insulin on in vitro leptin secretion by adipose tissue

Although it is widely accepted that insulin stimulates leptin secretion, a dual action was observed using a validated in vitro system, i.e., an early (less than 48 h) inhibitory action, followed later (48-96 h) by a clear-cut stimulation. While the inhibitory phase was observed at every glucose concentration tested (from 1 to 25 mM), the stimulatory phase required the presence of physiological or supraphysiological glucose concentrations. In fact, leptin secretion was virtually eliminated in the presence of glucose uptake inhibitors. This dual effect of insulin was not due to modifications of the ob mRNA levels, suggesting that it depends entirely on posttranslational mechanisms. In conclusion, insulin appears to induce an early inhibition of leptin secretion by the adipose cell, followed later by a stimulatory effect secondary to the metabolic changes triggered by the insulin-induced increase in glucose uptake.

Neuroendocrine consequences of fasting in adult male macaques: effects of recombinant rhesus macaque leptin infusion

Fasting inhibits the gonadotropic axis and stimulates the corticotropic and somatotropic axes. Since leptin is a product of fat cells that has been implicated in the control of both reproduction and metabolism, we hypothesized that the decrease in leptin observed during fasting was responsible for these effects on reproductive and metabolic hormones. Recombinant rhesus leptin (rrhLep) produced in our laboratory was infused (100 microgram/h) into fasted adult male rhesus macaques (6-9 kg) beginning at midnight after the first missed meal and continuing until the end of the study. Bioactive luteinizing hormone (LH), testosterone, cortisol and growth hormone (GH) were measured in plasma from samples collected at 15-min intervals for the last 15 h (42-57 h) of the fast. We analyzed pulsatile LH and GH secretion by deconvolution analysis and the orderliness of pulsatile LH and GH release by the approximate entropy (ApEn) statistic. There was no difference in LH pulse frequency between control and fasted groups, but there was a significant decrease in the mean concentration of LH released (7.6 +/- 1.4 ng/ml control vs. 2.7 +/- 0.65 ng/ml fasted) that was not relieved with rrhLep infusions (2.8 +/- 0.83 ng/ml). Model-free Cluster analysis confirmed these inferences and also indicated that the peak height was lower in the fasted (4.6 +/- 1.0 ng/ml) and the fasted + rrhLep (2.85 +/- 1.0 ng/ml) groups compared to controls (16. 3 +/- 1.4 ng/ml). Testosterone levels reflected those of LH. Fasting resulted in an increase in GH secretory pulse frequency (5.3 +/- 0. 95 pulses/15 h control vs. 12.8 +/- 1.4 pulses/15 h fasted) and this increase was not affected by rrhLep infusion (12.5 +/- 1.4 pulses/15 h). In addition, fasting also increased the ApEn (decreased the orderliness) of pulsatile GH secretion, and this characteristic was not relieved with rrhLep infusions. Cortisol levels in fasted animals were 2- to 3-fold higher than those observed in control studies, and this increase was particularly pronounced at the time when the animals expected their first meal of the day. The increase in circulating cortisol observed in fasted animals was not affected by rrhLep infusion. Glucose levels at the end of the sampling period were 80 mg/dl in controls, 48 mg/dl in fasted animals and 58 mg/dl in the fasted + rrhLep group. Circulating leptin levels averaged 1.2 +/- 0.37 ng/ml in control animals, 0.7 +/- 0.2 ng/ml in fasted animals and 10.1 +/- 5.6 ng/ml in fasted animals infused with rrhLep. These studies suggest that intravenous replacement with homologous leptin does not reverse the acute changes in GH, LH and cortisol secretion observed with fasting in the adult male macaque.

A Stat1alpha factor regulates the expression of the human vimentin gene by IFN-gamma

Vimentin is an intermediate filament protein normally expressed in cells of mesenchymal origin. Here, we report an increase in vimentin gene transcription induced by the cytokine interferon-y (IFN-gamma). Northern blot analysis and reporter gene assays reveal that IFN-gamma induces vimentin gene transcription in HeLa cells. However, no increase in vimentin mRNA synthesis was observed de novo in MCF-7 cells, which do not already express vimentin. Band shift analysis shows that the Stat1alpha protein mediates vimentin induction by IFN-gamma. A human mutant fibroblast cell line (U3A), which lacks Stat1alpha but expresses vimentin mRNA, yields no increase in vimentin mRNA levels on the addition of IFN-gamma. These results suggest that the induction of vimentin gene expression might be an important part of a complex cellular response to IFN-gamma.

The chicken leptin gene: has it been cloned?

The DNA sequence of a chicken leptin gene that shares 95% nucleotide similarity with the mouse leptin sequence has been recently reported (Taouis et al., 1998, Gene 208, 239-242). Experiments have been performed independently in two laboratories to try to confirm this finding. Fourteen PCR primers based on the mouse leptin sequence were designed to amplify the avian leptin gene. Four of the primers were identical to the mouse and published chicken leptin sequences. PCR amplification was carried out on genomic DNA and reverse-transcribed mRNA from the fat, liver, and pancreas of several chicken strains and from the domestic turkey, goose, and Japanese quail. No PCR products sharing close similarity to the mouse leptin sequence were generated from any avian templates. Amplification of mouse leptin sequence was consistently obtained when control mouse templates were used. Northern hybridization using a mouse leptin probe failed to produce a signal with poly(A)+ RNA from chicken fat and liver and from the fat and liver of force-fed geese but a strong signal was obtained from control mouse fat total RNA. Southern hybridization under low stringency washing conditions revealed hybridization of a mouse leptin probe to chicken genomic DNA. Under higher stringency washing conditions, the chicken signal disappeared, while those from control mouse and sheep genomic DNA remained. This suggests that the putative chicken leptin sequence shares less than the 83% nucleotide sequence identity between the mouse and sheep genes. It is concluded that a chicken leptin gene sequence with close sequence similarity to mouse leptin is not present in the chicken genome. Furthermore, mRNA sharing high sequence identity with mouse leptin is not present in the fat or liver of the domestic chicken, turkey, goose, or Japanese quail.

Monkey leptin receptor mRNA: sequence, tissue distribution, and mRNA expression in the adipose tissue of normal, hyperinsulinemic, and type 2 diabetic rhesus monkeys

OBJECTIVE

We have cloned the rhesus monkey leptin receptor and examined its mRNA expression levels in the adipose tissue of monkeys to investigate the regulation of gene expression of the leptin receptor.

RESEARCH METHODS AND PROCEDURES

Monkey leptin receptor cDNA was cloned by reverse transcriptase-polymerase chain reaction (RT-PCR). Tissue distribution of monkey leptin receptor was examined by Northern blot analysis and RT-PCR. The mRNA levels of monkey leptin receptor in adipose tissue of normal (n=10), hyperinsulinemic obese (n=8), and type 2 diabetic monkeys (n=8) were measured by quantitative RT-PCR.

RESULTS

Monkey leptin receptor cDNA had at least two alternatively spliced isoforms (long and short forms). The long form of the leptin receptor mRNA was expressed relatively highly in liver, adipose tissue, hypothalamus, and choroid plexus, whereas the total leptin receptors were expressed in every tissue examined. The mRNA levels of the long form of the leptin receptor in adipose tissue were not correlated to body weight, fasting plasma insulin, plasma glucose, or plasma leptin levels. The mRNA levels of the long form of the leptin receptor were highly correlated to that of the total leptin receptor (long and short form).

DISCUSSION

The long form of leptin receptor mRNA existed in adipose tissue as well as in liver and hypothalamus, suggesting that the leptin receptor in adipose tissue may be functional in adipose tissue. The expression of the leptin receptor mRNA in adipose tissue is not affected by obesity, hyperinsulinemia, or diabetes.

A biphasic developmental pattern of circulating leptin in the male rhesus macaque (Macaca mulatta)

To help elucidate the physiological role of leptin during somatic and sexual maturation, circulating concentrations of leptin were measured in 36 male rhesus monkeys of ages ranging from 0-20 yr. The body weight of these animals showed a steady increase of approximately 1 kg/yr during the first decade of life and reached a plateau at approximately 13 yr. In contrast, serum leptin concentrations showed a biphasic developmental pattern, which was highlighted by a strong negative correlation with body weight (r = -0.74, P < 0.001) before the onset of puberty (at approximately 3.5 yr) and by a strong positive correlation afterward (r = 0.77, P < 0.001). Overall, the developmental changes in serum leptin concentrations closely mimicked the expected developmental changes in serum testosterone concentrations (r = 0.62, P < 0.001), which were highly elevated at birth, fell to basal levels during the juvenile phase of development, and gradually rose again after the initiation of puberty. However, mean serum leptin concentrations during the peripubertal period itself (3-5 yr) were significantly lower (P < 0.01) than those observed during the first year of life or those observed in fully mature adults (i.e. > 7 yr) (3.5 +/- 0.3, 1.4 +/- 0.2, and 3.3 +/- 0.6 ng/ml, respectively). These data demonstrate that the role of leptin in energy homeostasis of primates is more than a simple linear relationship, being highly dependent upon the developmental age. Furthermore, the data do not support the hypothesis that leptin plays a major role in triggering the onset of puberty in primates, although the strong correlation between serum concentrations of leptin and testosterone suggests that the secretion of these two hormones may be causally linked.

A leptin dose-response study in obese (ob/ob) and lean (+/?) mice

This experiment determined the amount of leptin required to correct different abnormalities in leptin-deficient ob/ob mice. Baseline food intakes and body weights of lean (+/?) and obese (ob/ob) C57B1/6J <ob> mice were recorded for 7 days. An Alzet miniosmotic pump was placed in the peritoneal cavity of each mouse and delivered 0, 1, 2, 5, 10, or 42 microg/day human leptin for 7 days. In ob/ob mice, 2 microg leptin/day reduced food intake and body weight, and increased hypothalamic and brain stem serotonin concentrations. All fat pads were reduced 35-40% by 10 microg leptin/day, and liver weight, lipid, and glycogen decreased. Serum insulin and glucose were reduced in all leptin-treated ob/ob mice, and levels were normalized by 10 microg/day leptin. Low rectal temperatures of ob/ob mice were corrected by 10 and 42 microg/day leptin. These doses also increased brown adipose tissue uncoupling protein expression. The only responses in lean mice were a transient reduction in food intake and weight loss with 10 or 42 microg/day leptin. This study shows enhanced leptin sensitivity in ob/ob mice and suggests that increased temperature and sympathetic activity are indirect responses to high concentrations of protein.

Insulin stimulates both leptin secretion and production by rat white adipose tissue

Leptin, the peptide encoded by the obese gene, is secreted by adipose cells and plays a role in regulating food intake, energy expenditure, and adiposity. Because earlier studies suggested that insulin increases the expression of leptin, we investigated the effect of insulin on leptin secretion by adipose tissue. Epididymal fat pads were incubated in vitro in the presence or absence of insulin over a 4-h time course. Insulin increased leptin secretion by about 80% at all time points studied. After 10 min of insulin treatment, the amount of tissue-associated leptin was lower in insulin-stimulated tissue, presumably due to the increased secretion. At later times, both tissue-associated leptin and total leptin production were higher in insulin-treated tissue. In untreated, isolated adipose cells, immunostaining of leptin was detected in the endoplasmic reticulum by confocal microscopy. After insulin treatment, there were two populations of cells. In many cells, leptin staining became fainter and was restricted to a narrow band near the plasma membrane. However, in other cells the leptin-staining pattern was unchanged. Leptin did not colocalize with GLUT4, the glucose transporter isoform found primarily in insulin-responsive cells, in either basal or insulin-stimulated adipose cells. In this study, insulin increased both secretion and production of leptin by adipose tissue fragments. Interestingly, insulin appeared to stimulate the transport of leptin from the endoplasmic reticulum rather than acting on a pool of regulated secretory vesicles. (Endocrinology 138: 4463-4472, 1997)