Circadian variation of serum leptin in healthy and diabetic men

Leptin, from the Greek leptos, meaning thin (in reference to its ability to reduce body fat stores), is a hormone secreted primarily by adipocytes. At one time, leptin was portrayed as a potential means of combating obesity. Recently, leptin has been identified as a potent inhibitor of bone formation, acting through the central nervous system. Since numerous studies clearly show that bone remodeling is circadian rhythmic with peak activity during sleep, it is of interest to explore circadian variability in serum leptin. Accordingly, circadian characteristics of serum leptin were examined in 7 clinically healthy men and 4 obese men with type II diabetes. Blood samples were collected for 24 h at 3 h intervals beginning at 19:00. The dark (sleep) phase of the light-dark cycle extended from 22:30 to 06:30, with brief awakening for sampling at 01:00 and 04:00. Subjects consumed general hospital meals (2400 calories) at 16:30, 07:30, and 13:30. Serum leptin levels were determined by a R&D Systems enzyme immunoassay technique. Data were analyzed by linear least-squares estimation using the population multiple components method. A statistically significant (P < .018) circadian rhythm modeled by a single 24 h cosine curve characterized the data of each group. The 24 h mean leptin level was statistically greater (P < .001) in the obese diabetic men than in the healthy men (9.47 +/- 0.66 ng/mL vs. 24.07 +/- 1.71 ng/mL, respectively). Higher leptin levels occurred between midnight and roughly 02:30, and lowest leptin levels occurred between noon and the early afternoon. The phasing of this rhythm is similar to the circadian rhythm in bone remodeling previously described. Our results suggest the findings from a single morning blood sampling for leptin may be misleading since it may underestimate the mean 24 h and peak concentrations of the hormone.     

间歇性低氧对肥胖小鼠瘦素及其受体表达的影响

为探讨适度低氧环境对体重的影响及其作用机制,明确瘦素在其中的作用,用高脂饮食建立小鼠肥胖模型并观察间歇性低氧的干预效果。健康昆明小鼠随机分为4组（每组20只）,正常对照组：喂正常食物,不进行间歇性低氧训练;低氧组：喂正常食物,并进行间歇性低氧训练;肥胖组：喂高脂、高糖食物,但不进行间歇性低氧训练;低氧＋肥胖组,喂高脂、高糖食物,并进行间歇性低氧训练。40d后,测量小鼠体重,用酶联免疫吸附法测定血清瘦素水平,免疫组织化学检测肝脏瘦素受体表达,苏丹Ⅲ染色检测肝脏脂肪细胞分布和密度。结果显示,与正常对照组相比,肥胖组小鼠平均体重和平均血清瘦素水平显著升高,肝脏分布大量脂肪细胞,提示高脂模型建立成功;经过间歇性低氧训练后,低氧组和低氧＋肥胖组小鼠的平均体重及肝脏脂肪细胞分布密度和范围分别较对照组和肥胖组低,而血清瘦素水平明显增高;低氧＋肥胖组小鼠肝脏瘦素受体的表达高于肥胖组。结果提示,适度的间歇性低氧可以通过提高血清瘦素水平和增强肝脏瘦素受体表达而使体重减轻,并有效防止肝细胞脂肪变。     

Nutritional modulation of leptin expression and leptin action in obesity and obesity-associated complications

In obesity, an elevated accumulation and dysregulation of adipose tissue, due to an imbalance between energy intake and energy expenditure, usually coexists with the loss of responsiveness to leptin in central nervous system, and subsequently with hyperleptinemia. Leptin, a peptide hormone mainly produced by white adipose tissue, regulates energy homeostasis by stimulating energy expenditure and inhibiting food intake. Human obesity is characterized by increased plasma leptin levels, which have been related with different obesity-associated complications, such as chronic inflammatory state (risk factor for diabetes, cardiovascular and autoimmune diseases), as well as infertility and different types of cancer. Besides, leptin is also produced by placenta, and high leptin levels during pregnancy may be related with some pathological conditions such as gestational diabetes. This review focuses on the current insights and emerging concepts on potentially valuable nutrients and food components that may modulate leptin metabolism. Notably, several dietary food components, such as phenols, peptides, and vitamins, are able to decrease inflammation and improve leptin sensitivity by up- or down-regulation of leptin signaling molecules. On the other hand, some food components, such as saturated fatty acids may worsen chronic inflammation increasing the risk for pathological complications. Future research into nutritional mechanisms that restore leptin metabolism and signals of energy homeostasis may inspire new treatment options for obesity-related disorders.     

Circadian phase in adults of contrasting ages

There is evidence that aging may impair phase-shifting responses to light synchronizers, which could lead to disturbed or malsynchronized circadian rhythms. To explore this hypothesis, 62 elder participants (age, 58 to 84 years) and 25 young adults (age, 19 to 40 years) were studied, first with baseline 1-wk wrist actigraphy at home and then by 72 h in-laboratory study using an ultra-short sleep-wake cycle. Subjects were awake for 60 minutes in 50 lux followed by 30 minutes of darkness for sleep. Saliva samples were collected for melatonin, and urine samples were collected for aMT6s (a urinary metabolite of melatonin) and free cortisol every 90 minutes. Oral temperatures were also measured every 90 minutes. The timing of the circadian rhythms was not significantly more variable among the elders. The times of lights-out and wake-up at home and urinary free cortisol occurred earlier among elders, but the acrophases (cosinor analysis-derived peak time) of the circadian rhythm of salivary melatonin, urinary aMT6s, and oral temperature were not significantly phase-advanced among elders. The estimated duration of melatonin secretion was 9.9 h among elders and 8.4 h among young adults (p < 0.025), though the estimated half-life of blood melatonin was shorter among elders (p < 0.025), and young adults had higher saliva melatonin and urinary aMT6s levels. In summary, there was no evidence for circadian desynchronization associated with aging, but there was evidence of some rearrangement of the internal phase-angles among the studied circadian rhythms.     

Circadian rhythm of blood leptin level in obese and non-obese people

Leptin, an adipose tissue hormone, has circadian variations in its secretion. Aims of this study were to show how circadian rhythm depends on fat tissue distribution in obese and non-obese subjects. The research was carried out on 70 subjects (37 men and 33 women) with an average body mass index (BMI) of 25.22 kg/m2. Concentration of leptin in blood was measured at 8.30 a.m., 12.30 p.m. and 6.30 p.m. Basal leptin level correlated strongly with all isolated regions of subcutaneous fat tissue in women and obese subjects. Circadian changes of blood leptin level in non-obese people are more significant than these changes in obese people. Differences in circadian pattern of leptin secretion between obese and non-obese subjects were probably caused by enlarged volume of subcutaneous fat tissue in obese people. Lean subjects have subcutaneous fat in physiological range which allows influence of some hormones (insulin or cortizol) or food intake on leptin secretion.     

Circadian phase variation in bipolar I disorder

Abnormalities in circadian rhythms are prominent features of bipolar I disorder (BD1). To investigate circadian variation in BD1, we evaluated morningness-eveningness (M/E), a stable trait reflecting circadian phase, using the composite scale (CS) among BD1 patients (DSM IV criteria; n=75), unscreened controls (n=349), and patients with schizophrenia (SZ) or schizoaffective disorder (SZA) (n=81). Our analyses showed that CS scores correlated significantly with age but did not differ by gender among the controls. BD1 patients differed significantly from controls and from SZ/SZA patients when age was considered. CS scores were distributed bi-modally among BD1 cases. There are several possible reasons for the observed heterogeneity. Younger BD1 patients, and those with rapid mood swings, were significantly more likely to have lower CS scores (i.e., to score in the 'evening' range and to have later circadian phase). CS scores were also positively correlated with the age at onset and the duration of the most severe depressive episodes. These relationships were not observed among the SZ/SZA groups. Thus, distinct patterns of M/E were noted among BD1 patients and among BD1 subgroups. The impact of medication, mood state, and chronicity on CS scores needs to be considered.     

Mechanisms mediating renal sympathetic activation to leptin in obesity

Leptin plays a critical role in the control of energy homeostasis. The sympathetic cardiovascular actions of leptin have emerged as a potential link between obesity and hypertension. We previously demonstrated that in mice, modest obesity induced by 10 wk of a high-fat diet is associated with preservation of leptin ability to increase renal sympathetic nerve activity (SNA) despite the resistance to the metabolic effects of leptin. Here, we examined whether selective leptin resistance exists in mice with late-stage diet-induced obesity (DIO) produced by 20 wk of a high-fat diet. The decrease in food intake and body weight induced by intraperitoneal or intracerebroventricular injection of leptin was significantly attenuated in the DIO mice. Regional SNA responses to intravenous leptin were also attenuated in DIO mice. In contrast, intracerebroventricularly administered leptin caused contrasting effects on regional SNA in DIO mice. Renal SNA response to intracerebroventricular leptin was preserved, whereas lumbar and brown adipose tissue SNA responses were attenuated. Intact renal SNA response to leptin combined with the increased cerebrospinal fluid leptin levels in DIO mice represents a potential mechanism for the adverse cardiovascular consequences of obesity. Lastly, we examined the role of phosphoinositol-3 kinase (PI3K) and melanocortin receptors (MCR) in mediating the preserved renal SNA response to leptin in obesity. Presence of PI3K inhibitor (LY294002) or MC3/4R antagonist (SHU9119) significantly attenuated the renal SNA response to leptin in DIO and agouti obese mice. Our results demonstrate the importance of PI3K and melanocortin receptors in the transduction of leptin-induced renal sympathetic activation in obesity.     

Reduced central leptin sensitivity in rats with diet-induced obesity

On low-fat chow diet, rats prone to diet-induced obesity (DIO) have increased arcuate nucleus neuropeptide Y (NPY) expression but similar leptin levels compared with diet-resistant (DR) rats (19). Here, body weight and leptin levels rose in DIO rats, and they defended their higher body weight after only 1 wk on a 31% fat high-energy (HE) diet. However, DIO NPY expression did not fall to DR levels until 4 wk when plasma leptin was 168% of DR levels. When switched to chow, DIO rats lost carcass fat (18). By 10 wk, leptin levels fell to 148% and NPY expression again rose to 150% of DR levels. During 4 wk of food restriction, DIO leptin fell by approximately 50% while NPY increased by 30%. While both returned to control levels by 8 wk, DIO rats still regained all lost weight when fed ad libitum. Finally, the anorexic effect of intracerebroventricular leptin (10 microg) was inversely correlated with subsequent 3-wk weight gain on HE diet. Thus NPY expression and food intake are less sensitive to the leptin's suppressive effects in DIO rats. While this may predispose them to develop DIO, it does not fully explain their defense of a higher body weight on HE diet.     

Impaired transport of leptin across the blood-brain barrier in obesity

Leptin is a 17-kDa protein secreted by fat cells that regulates body adiposity by crossing the blood-brain barrier (BBB) to affect feeding and thermogenesis. Obese human and rodent models of dietary obesity have shown decreased sensitivity to blood-borne leptin, postulated to be due to impaired transport of leptin across the BBB. We show here that the transport rate of leptin across the BBB is reduced about 2/3 in 12-month-old obese CD-1 mice. In a follow-up study, a perfusion method was used that replaced the blood with a buffer containing low concentrations of radioactive leptin. Obese mice still had lower rates of transport into the brain than lean mice, which shows that the reduction in transport rate associated with obesity is not due simply to saturation of transporter secondary to higher serum leptin levels as has been thought, but to a decreased capacity of the BBB to transport leptin. This suggests a new model for obesity in which a defect in the BBB transport of leptin into the CNS underlies the insensitivity to leptin and leads to obesity.     

Circadian photoentrainment: parameters of phase delaying

Experiments were carried out using simulated den cages to delineate specific characteristics of phase delaying in circadian photoentrainment of a nocturnal rodent, the flying squirrel. The principal experiments entailed presentation of one to five consecutive 15-min white-light pulses per activity cycle at activity onset to animals free-running in darkness, in order to determine the immediate and final phase-shifting effect. Auxiliary experiments recorded entrainment patterns on light-dark (LD) schedules in the den cages. Phase response curves (PRCs) based on 15-min white-light pulses in standard wheel cages were also constructed for these animals as background information for interpreting the phase-delaying experiments. Exposure of a den animal to light by light sampling at the time of initial arousal from the rest state at circadian time (CT) 12, either by an LD schedule or by a 15-min light pulse, resulted in a return to the nest box for a short rest period. The phase delay occurring after a single light exposure at activity onset was equal to the induced rest, thus suggesting an immediate phase shift. The maximum delay was about 1 1/2 hr/cycle, with the amount of delay related to the number of light exposures. During the photoentrained state on an LD schedule, the activity rhythm of a den-housed animal was essentially free-running on the days following a phase delay. The data are used to expand current models for photoentrainment of circadian activity rhythms in nocturnal rodents.     

Common genetic components of obesity traits and serum leptin

To estimate common and distinct genetic influences on a panel of obesity-related traits and serum leptin level in adults. In a cross-sectional study of 625 Danish, adult, healthy, monozygotic, and same-sex dizygotic twin pairs of both genders, we carried out detailed anthropometry (height, weight, waist and hip, and skin-fold thickness, body composition assessment by bioimpedance (fat mass and fat-free mass), and measurement of serum leptin level. Bivariate variance component analyses estimated the additive genetic correlations between these measurements. The genetic correlations between the traits for overall fatness (BMI and fat mass index, kg/m(2)) were 0.94 in men and 0.98 in women, and their correlations with the various local fatness measures ranged from 0.49 to 0.83 in men and from 0.70 to 0.87 in women. The correlations between the truncal measures (waist circumference and truncal skin folds) and between the peripheral measures (hip circumference and peripheral skin folds) were 0.57 and 0.47 in men and 0.71 and 0.70 in women, respectively. The correlations between the truncal and peripheral measures ranged between 0.49 and 0.72 in men and between 0.61 and 0.82 in women. For leptin vs. the various measures of overall and local fatness the correlations ranged from 0.54 to 0.74 in men and from 0.48 to 0.75 in women. All correlations were significantly <1.00. The study supports control of overall fat mass and peripheral and truncal fat mass by both shared and different genetic components, which suggests that it is important to distinguish between the different phenotypes in the search for genes involved in the development of obesity.     

Elevated hypothalamic TCPTP in obesity contributes to cellular leptin resistance

In obesity, anorectic responses to leptin are diminished, giving rise to the concept of "leptin resistance." Increased expression of protein tyrosine phosphatase 1B (PTP1B) has been associated with the attenuation of leptin signaling and development of cellular leptin resistance. Here we report that hypothalamic levels of the tyrosine phosphatase TCPTP are also elevated in obesity to attenuate the leptin response. We show that mice that lack TCPTP in neuronal cells have enhanced leptin sensitivity and are resistant to high-fat-diet-induced weight gain and the development of leptin resistance. Also, intracerebroventricular administration of a TCPTP inhibitor enhances leptin signaling and responses in mice. Moreover, the combined deletion of TCPTP and PTP1B in neuronal cells has additive effects in the prevention of diet-induced obesity. Our results identify TCPTP as a critical negative regulator of hypothalamic leptin signaling and causally link elevated TCPTP to the development of cellular leptin resistance in obesity.     

Insulin and leptin relations in obesity: a multimedia approach

Obesity has been recognized as a worldwide public health problem. It significantly increases the chances of developing several diseases, including Type II diabetes. The roles of insulin and leptin in obesity involve reactions that can be better understood when they are presented step by step. The aim of this work was to design software with data from some of the most recent publications on obesity, especially those concerning the roles of insulin and leptin in this metabolic disturbance. The most notable characteristic of this software is the use of animations representing the cellular response together with the presentation of recently discovered mechanisms on the participation of insulin and leptin in processes leading to obesity. The software was field tested in the Biochemistry of Nutrition web-based course. After using the software and discussing its contents in chatrooms, students were asked to answer an evaluation survey about the whole activity and the usefulness of the software within the learning process. The teaching assistants (TA) evaluated the software as a tool to help in the teaching process. The students' and TAs' satisfaction was very evident and encouraged us to move forward with the software development and to improve the use of this kind of educational tool in biochemistry classes.     

Role of premature leptin surge in obesity resulting from intrauterine undernutrition

Intrauterine undernutrition is closely associated with obesity related to detrimental metabolic sequelae in adulthood. We report a mouse model in which offspring with fetal undernutrition (UN offspring), when fed a high-fat diet (HFD), develop pronounced weight gain and adiposity. In the neonatal period, UN offspring exhibited a premature onset of neonatal leptin surge compared to offspring with intrauterine normal nutrition (NN offspring). Unexpectedly, premature leptin surge generated in NN offspring by exogenous leptin administration led to accelerated weight gain with an HFD. Both UN offspring and neonatally leptin-treated NN offspring exhibited an impaired response to acute peripheral leptin administration on a regular chow diet (RCD) with impaired leptin transport to the brain as well as an increased density of hypothalamic nerve terminals. The present study suggests that the premature leptin surge alters energy regulation by the hypothalamus and contributes to “developmental origins of health and disease.”     

The influence of leptin on early life programming of obesity

Epidemiological evidence together with experimental models shows a direct relationship between fetal and early postnatal growth patterns and an increased risk of adult metabolic disease. Maternal health and nutrition are key determinants in influencing infant growth but the precise molecular mechanisms underlying this relationship are unclear, although it is evident that there are critical time windows when these effects are important. Animal models show mechanistic parallels with human populations and highlight that the early environment represents a therapeutic window for protection from obesity and metabolic disease. The observation that developmental programming can be reversed has been demonstrated in studies in which both maternal and neonatal leptin treatment prevents the induction of the adverse metabolic phenotype. Given that orally administered peptides are absorbed intact by the new born, the prospect of providing supplemental leptin either as drops or in milk deserves serious consideration as a means of reducing or reversing the obesity and type 2 diabetes epidemic.     

Effects of leptin and obesity on the upper airway function

Obesity is associated with alterations in upper airway collapsibility during sleep. Obese, leptin-deficient mice demonstrate blunted ventilatory control, leading us to hypothesize that (1) obesity and leptin deficiency would predispose to worsening neuromechanical upper airway function and that (2) leptin replacement would acutely reverse neuromuscular defects in the absence of weight loss. In age-matched, anesthetized, spontaneously breathing C57BL/6J (BL6) and ob(-)/ob(-) mice, we characterized upper airway pressure-flow dynamics during ramp decreases in nasal pressure (P(N)) to determine the passive expiratory critical pressure (P(CRIT)) and active responses to reductions in P(N), including the percentage of ramps showing inspiratory flow limitation (IFL; frequency), the P(N) threshold at which IFL developed, maximum inspiratory airflow (Vi(max)), and genioglossus electromyographic (EMG(GG)) activity. Elevations in body weight were associated with progressive elevations in P(CRIT) (0.1 ± 0.02 cmH(2)O/g), independent of mouse strain. P(CRIT) was also elevated in ob(-)/ob(-) compared with BL6 mice (1.6 ± 0.1 cmH(2)O), independent of weight. Both obesity and leptin deficiency were associated with significantly higher IFL frequency and P(N) threshold and lower VI(max). Very obese ob(-)/ob(-) mice treated with leptin compared with nontreated mice showed a decrease in IFL frequency (from 63.5 ± 2.9 to 30.0 ± 8.6%) and P(N) threshold (from -0.8 ± 1.1 to -5.6 ± 0.8 cmH(2)O) and increase in VI(max) (from 354.1 ± 25.3 to 659.0 ± 71.8 μl/s). Nevertheless, passive P(CRIT) in leptin-treated mice did not differ significantly from that seen in nontreated ob(-)/ob(-) mice. The findings suggest that weight and leptin deficiency produced defects in upper airway neuromechanical control and that leptin reversed defects in active neuromuscular responses acutely without reducing mechanical loads.     

Impaired activation of phosphatidylinositol 3-kinase by leptin is a novel mechanism of hepatic leptin resistance in diet-induced obesity

Obesity is associated with the development of leptin resistance. However, the effects of leptin resistance on leptin-regulated metabolic processes and the biochemical defects that cause leptin resistance are poorly understood. We have addressed in rats the effect of dietinduced obesity (DIO), a situation of elevated tissue lipid levels, on the well described lipid-lowering effect of leptin in liver, an action that is proposed to be important for the prevention of tissue lipotoxicity and insulin resistance. In addition, we have addressed the role of phosphatidylinositol 3-kinase (PI 3-kinase) in mediating the acute effects of leptin on hepatic lipid levels in lean and DIO animals. A 90-min leptin ( approximately 10 ng/ml) perfusion of isolated livers from lean animals decreased triglyceride levels by 42 +/- 5% (p = 0.006). However, leptin concentrations ranging from approximately 10 to approximately 90 ng/ml had no effect on triglyceride levels in livers from DIO animals. The acute lipid-lowering effect of leptin on livers from lean animals was mediated by a PI 3-kinase-dependent mechanism, because wortmannin and LY294002, the PI 3-kinase inhibitors, blocked the effects of leptin on hepatic triglyceride levels and leptin increased liver PI 3-kinase activity by 183 +/- 6% (p = 0.003) and insulin receptor substrate 1 tyrosine phosphorylation by 185 +/- 30% (p = 0.02) in the absence of PI 3-kinase inhibitors. Contrary to the effects of leptin in lean livers, leptin did not activate PI 3-kinase in livers from DIO rats. These data present evidence for a role for 1). leptin resistance in contributing to the excessive accumulation of tissue lipid in obesity, 2). PI 3-kinase in mediating the acute lipid-lowering effects of leptin in liver, and 3). defective leptin activation of PI 3-kinase as a novel mechanism of leptin resistance.